When including a file (only once, garentied by a "if (defined(tag)) return; define(tag, TRUE);"), an error occurs saying :
"Cannot redeclare function() [actually, the 1st function defined in the file] in Unknown line <line>"
The name of the file does not even appear in the message (Unknown instead).

I'm working on a project that was very well running with version 4.0.1pl2 (I am going to reinstall it by the way !) ...

Configure line : --with-apxs=/usr/local/apache/bin/apxs --with-mysql=/usr/local/mysql --with-mm --with-mcrypt --with-mhash

php.ini :
---------------------------------------------------------------------------------
[PHP]

;;;;;;;;;;;;;;;;;;;
; About this file ;
;;;;;;;;;;;;;;;;;;;

; This file controls many aspects of PHP's behavior.  In order for PHP to
; read it, it must be named 'php.ini'.  PHP looks for it in the current
; working directory, in the path designated by the environment variable
; PHPRC, and in the path that was defined in compile time (in that order).
; Under Windows, the compile-time path is the Windows directory.  The
; path in which the php.ini file is looked for can be overriden using
; the -c argument in command line mode.
;
; The syntax of the file is extremely simple.  Whitespace and Lines
; beginning with a semicolon are silently ignored (as you probably guessed).
; Section headers (e.g. [Foo]) are also silently ignored, even though
; they might mean something in the future.
;
; Directives are specified using the following syntax:
; directive = value
; Directive names are *case sensitive* - foo=bar is different from FOO=bar.
;
; The value can be a string, a number, a PHP constant (e.g. E_ALL or M_PI), one
; of the INI constants (On, Off, True, False, Yes, No and None) or an expression
; (e.g. E_ALL & ~E_NOTICE), or a quoted string ("foo").
;
; Expressions in the INI file are limited to bitwise operators and parentheses:
; |                             bitwise OR
; &                             bitwise AND
; ~                             bitwise NOT
;
; Boolean flags can be turned on using the values 1, On, True or Yes.
; They can be turned off using the values 0, Off, False or No.
;
; An empty string can be denoted by simply not writing anything after the equal
; sign, or by using the None keyword:
;
;   foo =                       ; sets foo to an empty string
;       foo = none              ; sets foo to an empty string
;       foo = "none"    ; sets foo to the string 'none'
;
; If you use constants in your value, and these constants belong to a dynamically
; loaded extension (either a PHP extension or a Zend extension), you may only
; use these constants *after* the line that loads the extension.
;
; All the values in the php.ini-dist file correspond to the builtin
; defaults (that is, if no php.ini is used, or if you delete these lines,
; the builtin defaults will be identical).

;;;;;;;;;;;;;;;;;;;;
; Language Options ;
;;;;;;;;;;;;;;;;;;;;

engine                  =       On      ; Enable the PHP scripting language engine under Apache
short_open_tag  =       On      ; allow the <? tag.  otherwise, only <?php and <script> tags are recognized.
asp_tags                =       Off ; allow ASP-style <% %> tags
precision               =       14      ; number of significant digits displayed in floating point numbers
y2k_compliance  =       Off     ; whether to be year 2000 compliant (will cause problems with non y2k compliant browsers)
output_buffering        = Off   ; Output buffering allows you to send header lines (including cookies)
                                                        ; even after you send body content, in the price of slowing PHP's
                                                        ; output layer a bit.
                                                        ; You can enable output buffering by in runtime by calling the output
                                                        ; buffering functions, or enable output buffering for all files
                                                        ; by setting this directive to On.
implicit_flush          = Off   ; Implicit flush tells PHP to tell the output layer to flush itself
                                                        ; automatically after every output block.  This is equivalent to
                                                        ; calling the PHP function flush() after each and every call to print()
                                                        ; or echo() and each and every HTML block.
                                                        ; Turning this option on has serious performance implications, and
                                                        ; is generally recommended for debugging purposes only.
allow_call_time_pass_reference  = On    ; whether to enable the ability to force arguments to be 
                                                                                ; passed by reference at function-call time.  This method
                                                                                ; is deprecated, and is likely to be unsupported in future
                                                                                ; versions of PHP/Zend.  The encouraged method of specifying
                                                                                ; which arguments should be passed by reference is in the
                                                                                ; function declaration.  You're encouraged to try and
                                                                                ; turn this option Off, and make sure your scripts work
                                                                                ; properly with it, to ensure they will work with future
                                                                                ; versions of the language (you will receive a warning
                                                                                ; each time you use this feature, and the argument will
                                                                                ; be passed by value instead of by reference).

; Safe Mode
safe_mode               =       Off
safe_mode_exec_dir      =
safe_mode_allowed_env_vars = PHP_                                       ; Setting certain environment variables
                                                                                                        ; may be a potential security breach.
                                                                                                        ; This directive contains a comma-delimited
                                                                                                        ; list of prefixes.  In Safe Mode, the
                                                                                                        ; user may only alter environment
                                                                                                        ; variables whose names begin with the
                                                                                                        ; prefixes supplied here.
                                                                                                        ; By default, users will only be able
                                                                                                        ; to set environment variables that begin
                                                                                                        ; with PHP_ (e.g. PHP_FOO=BAR).
                                                                                                        ; Note:  If this directive is empty, PHP
                                                                                                        ; will let the user modify ANY environment
                                                                                                        ; variable!
safe_mode_protected_env_vars = LD_LIBRARY_PATH          ; This directive contains a comma-
                                                                                                        ; delimited list of environment variables,
                                                                                                        ; that the end user won't be able to
                                                                                                        ; change using putenv().
                                                                                                        ; These variables will be protected
                                                                                                        ; even if safe_mode_allowed_env_vars is
                                                                                                        ; set to allow to change them.

; Colors for Syntax Highlighting mode.  Anything that's acceptable in <font color=???> would work.
highlight.string        =       #DD0000
highlight.comment       =       #FF8000
highlight.keyword       =       #007700
highlight.bg            =       #FFFFFF
highlight.default       =       #0000BB
highlight.html          =       #000000

; Misc
expose_php      =       On              ; Decides whether PHP may expose the fact that it is installed on the
                                                ; server (e.g., by adding its signature to the Web server header).
                                                ; It is no security threat in any way, but it makes it possible
                                                ; to determine whether you use PHP on your server or not.

;;;;;;;;;;;;;;;;;;;
; Resource Limits ;
;;;;;;;;;;;;;;;;;;;

max_execution_time = 30     ; Maximum execution time of each script, in seconds (UNIX only)
memory_limit = 8388608          ; Maximum amount of memory a script may consume (8MB)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Error handling and logging ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; error_reporting is a bit-field.  Or each number up to get desired error reporting level
; E_ALL                         - All errors and warnings
; E_ERROR                       - fatal run-time errors
; E_WARNING                     - run-time warnings (non fatal errors)
; E_PARSE                       - compile-time parse errors
; E_NOTICE                      - run-time notices (these are warnings which often result from a bug in
;                                         your code, but it's possible that it was intentional (e.g., using an
;                                         uninitialized variable and relying on the fact it's automatically
;                                         initialized to an empty string)
; E_CORE_ERROR          - fatal errors that occur during PHP's initial startup
; E_CORE_WARNING        - warnings (non fatal errors) that occur during PHP's initial startup
; E_COMPILE_ERROR       - fatal compile-time errors
; E_COMPILE_WARNING     - compile-time warnings (non fatal errors)
; Examples:
; error_reporting = E_ALL & ~E_NOTICE                                           ; show all errors, except for notices
; error_reporting = E_COMPILE_ERROR|E_ERROR|E_CORE_ERROR        ; show only errors
error_reporting =       E_ALL & ~E_NOTICE               ; Show all errors except for notices
display_errors  =       Off     ; Print out errors (as a part of the HTML script)
log_errors              =       On      ; Log errors into a log file (server-specific log, stderr, or error_log (below))
track_errors    =       On      ; Store the last error/warning message in $php_errormsg (boolean)
;error_prepend_string = "<font color=ff0000>"   ; string to output before an error message
;error_append_string = "</font>"                ; string to output after an error message
error_log       =       /var/log/apache_PHP     ; log errors to specified file
;error_log      =       syslog          ; log errors to syslog (Event Log on NT, not valid in Windows 95)
warn_plus_overloading   =       Off             ; warn if the + operator is used with strings

;;;;;;;;;;;;;;;;;
; Data Handling ;
;;;;;;;;;;;;;;;;;

variables_order         =       "EGPCS" ; This directive describes the order in which PHP registers
                                                                ; GET, POST, Cookie, Environment and Built-in variables (G, P,
                                                                ; C, E & S respectively, often referred to as EGPCS or GPC).
                                                                ; Registration is done from left to right, newer values override
                                                                ; older values.
register_globals        =       On              ; Whether or not to register the EGPCS variables as global
                                                                ; variables.  You may want to turn this off if you don't want
                                                                ; to clutter your scripts' global scope with user data.  This makes
                                                                ; most sense when coupled with track_vars - in which case you can
                                                                ; access all of the GPC variables through the $HTTP_*_VARS[],
                                                                ; variables.
register_argv_argc      =       On              ; This directive tells PHP whether to declare the argv&argc
                                                                ; variables (that would contain the GET information).  If you
                                                                ; don't use these variables, you should turn it off for
                                                                ; increased performance
track_vars                      =       On              ; enable the $HTTP_*_VARS[] arrays, where * is one of
                                                                ; ENV, POST, GET, COOKIE or SERVER.
gpc_order                       =       "GPC"   ; This directive is deprecated.  Use variables_order instead.

; Magic quotes
magic_quotes_gpc        =       Off             ; magic quotes for incoming GET/POST/Cookie data
magic_quotes_runtime=   Off             ; magic quotes for runtime-generated data, e.g. data from SQL, from exec(), etc.
magic_quotes_sybase     =       Off             ; Use Sybase-style magic quotes (escape ' with '' instead of \')

; automatically add files before or after any PHP document
auto_prepend_file       =
auto_append_file        =

; As of 4.0b4, PHP always outputs a character encoding by default in
; the Content-type: header.  To disable sending of the charset, simply
; set it to be empty.
; PHP's built-in default is text/html
default_mimetype = "text/html"
;default_charset = "iso-8859-1"

;;;;;;;;;;;;;;;;;;;;;;;;;
; Paths and Directories ;
;;;;;;;;;;;;;;;;;;;;;;;;;

include_path    =                   ; UNIX: "/path1:/path2"  Windows: "\path1;\path2"
doc_root                =                                       ; the root of the php pages, used only if nonempty
user_dir                =                                       ; the directory under which php opens the script using /~username, used only if nonempty
;upload_tmp_dir =                       ; temporary directory for HTTP uploaded files (will use system default if not specified)
upload_max_filesize = 2097152       ; 2 Meg default limit on file uploads
extension_dir   =       ./                              ; directory in which the loadable extensions (modules) reside

;;;;;;;;;;;;;;;;;;;;;;
; Dynamic Extensions ;
;;;;;;;;;;;;;;;;;;;;;;

; if you wish to have an extension loaded automaticly, use the
; following syntax:  extension=modulename.extension
; for example, on windows,
; extension=msql.dll
; or under UNIX,
; extension=msql.so
; Note that it should be the name of the module only, no directory information 
; needs to go here.  Specify the location of the extension with the extension_dir directive above.

; Windows Extensions
;extension=php_mysql.dll
;extension=php_nsmail.dll
;extension=php_calendar.dll
;extension=php_dbase.dll
;extension=php_filepro.dll
;extension=php_gd.dll
;extension=php_dbm.dll
;extension=php_mssql.dll
;extension=php_zlib.dll
;extension=php_filepro.dll
;extension=php_imap4r2.dll
;extension=php_ldap.dll
;extension=php_crypt.dll
;extension=php_msql2.dll
;extension=php_odbc.dll

;;;;;;;;;;;;;;;;;;;
; Module Settings ;
;;;;;;;;;;;;;;;;;;;

[Syslog]
define_syslog_variables = Off   ; Whether or not to define the various syslog variables,
                                                                ; e.g. $LOG_PID, $LOG_CRON, etc.  Turning it off is a
                                                                ; good idea performance-wise.  In runtime, you can define
                                                                ; these variables by calling define_syslog_variables()

[mail function]
SMTP                    =       localhost                       ;for win32 only
sendmail_from   =       me@localhost.com        ;for win32 only
;sendmail_path  =                                               ;for unix only, may supply arguments as well (default is sendmail -t)

[Debugger]
debugger.host   =       localhost
debugger.port   =       7869
debugger.enabled        =       False

[Logging]
; These configuration directives are used by the example logging mechanism.
; See examples/README.logging for more explanation.
;logging.method    = db
;logging.directory = /path/to/log/directory

[SQL]
sql.safe_mode   =       Off

[ODBC]
;uodbc.default_db               =       Not yet implemented
;uodbc.default_user             =       Not yet implemented
;uodbc.default_pw               =       Not yet implemented
uodbc.allow_persistent  =       On      ; allow or prevent persistent links
uodbc.check_persistent  =       On      ; check that a connection is still validbefore reuse
uodbc.max_persistent    =       -1      ; maximum number of persistent links. -1 means no limit
uodbc.max_links                 =       -1      ; maximum number of links (persistent+non persistent). -1 means no limit
uodbc.defaultlrl        =       4096    ; Handling of LONG fields. Returns number of bytes to variables, 0 means passthru
uodbc.defaultbinmode    =       1       ; Handling of binary data. 0 means passthru, 1 return as is, 2 convert to char
; See the documentation on odbc_binmode and odbc_longreadlen for an explanation of uodbc.defaultlrl
; and uodbc.defaultbinmode

[MySQL]
mysql.allow_persistent  =       On      ; allow or prevent persistent link
mysql.max_persistent    =       -1      ; maximum number of persistent links. -1 means no limit
mysql.max_links                 =       -1      ; maximum number of links (persistent+non persistent).  -1 means no limit
mysql.default_port              =               ; default port number for mysql_connect().  If unset,
                                                                ; mysql_connect() will use the $MYSQL_TCP_PORT, or the mysql-tcp
                                                                ; entry in /etc/services, or the compile-time defined MYSQL_PORT
                                                                ; (in that order).  Win32 will only look at MYSQL_PORT.
mysql.default_host              =               ; default host for mysql_connect() (doesn't apply in safe mode)
mysql.default_user              =               ; default user for mysql_connect() (doesn't apply in safe mode)
mysql.default_password  =               ; default password for mysql_connect() (doesn't apply in safe mode)
                                                                ; Note that this is generally a *bad* idea to store passwords
                                                                ; in this file.  *Any* user with PHP access can run
                                                                ; 'echo cfg_get_var("mysql.default_password")' and reveal that
                                                                ; password!  And of course, any users with read access to this
                                                                ; file will be able to reveal the password as well.

[mSQL]
msql.allow_persistent   =       On      ; allow or prevent persistent link
msql.max_persistent             =       -1      ; maximum number of persistent links. -1 means no limit
msql.max_links                  =       -1      ; maximum number of links (persistent+non persistent).  -1 means no limit

[PostgresSQL]
pgsql.allow_persistent  =       On      ; allow or prevent persistent link
pgsql.max_persistent    =       -1      ; maximum number of persistent links. -1 means no limit
pgsql.max_links                 =       -1      ; maximum number of links (persistent+non persistent).  -1 means no limit

[Sybase]
sybase.allow_persistent =       On      ; allow or prevent persistent link
sybase.max_persistent   =       -1      ; maximum number of persistent links. -1 means no limit
sybase.max_links                =       -1      ; maximum number of links (persistent+non persistent).  -1 means no limit
;sybase.interface_file  =       "/usr/sybase/interfaces"
sybase.min_error_severity       =       10      ; minimum error severity to display
sybase.min_message_severity     =       10      ; minimum message severity to display
sybase.compatability_mode       = Off   ; compatability mode with old versions of PHP 3.0.
                                                                        ; If on, this will cause PHP to automatically assign types to results
                                                                        ; according to their Sybase type, instead of treating them all as
                                                                        ; strings.  This compatability mode will probably not stay around
                                                                        ; forever, so try applying whatever necessary changes to your code,
                                                                        ; and turn it off.

[Sybase-CT]
sybct.allow_persistent  =       On              ; allow or prevent persistent link
sybct.max_persistent    =       -1              ; maximum number of persistent links. -1 means no limit
sybct.max_links                 =       -1              ; maximum number of links (persistent+non persistent).  -1 means no limit
sybct.min_server_severity       =       10      ; minimum server message severity to display
sybct.min_client_severity       =       10      ; minimum client message severity to display

[bcmath]
bcmath.scale    =       0       ; number of decimal digits for all bcmath functions

[browscap]
;browscap       =       extra/browscap.ini

[Informix]
ifx.default_host                =               ; default host for ifx_connect() (doesn't apply in safe mode)
ifx.default_user                =               ; default user for ifx_connect() (doesn't apply in safe mode)
ifx.default_password            =               ; default password for ifx_connect() (doesn't apply in safe mode)
ifx.allow_persistent            =       On      ; allow or prevent persistent link
ifx.max_persistent              =       -1      ; maximum number of persistent links. -1 means no limit
ifx.max_links                   =       -1      ; maximum number of links (persistent+non persistent).  -1 means no limit
ifx.textasvarchar               =       0       ; if set on, select statements return the contents of a text blob instead of it's id
ifx.byteasvarchar               =       0       ; if set on, select statements return the contents of a byte blob instead of it's id
ifx.charasvarchar               =       0       ; trailing blanks are stripped from fixed-length char columns. May help the life
                                                ; of Informix SE users. 
ifx.blobinfile                  =       0       ; if set on, the contents of text&byte blobs are dumped to a file instead of
                                                ; keeping them in memory
ifx.nullformat                  =       0       ; NULL's are returned as empty strings, unless this is set to 1. In that case,
                                                ; NULL's are returned as string 'NULL'.

[Session]
session.save_handler      = files   ; handler used to store/retrieve data
session.save_path         = /tmp    ; argument passed to save_handler
                                    ; in the case of files, this is the
                                    ; path where data files are stored
session.use_cookies       = 1       ; whether to use cookies
session.name              = PHPSESSID  
                                    ; name of the session
                                    ; is used as cookie name
session.auto_start        = 0       ; initialize session on request startup
session.cookie_lifetime   = 0       ; lifetime in seconds of cookie
                                    ; or if 0, until browser is restarted
session.cookie_path       = /       ; the path the cookie is valid for
session.cookie_domain     =         ; the domain the cookie is valid for
session.serialize_handler = php     ; handler used to serialize data
                                    ; php is the standard serializer of PHP
session.gc_probability    = 1       ; percentual probability that the 
                                    ; 'garbage collection' process is started
                                    ; on every session initialization
session.gc_maxlifetime    = 1440    ; after this number of seconds, stored
                                    ; data will be seen as 'garbage' and
                                    ; cleaned up by the gc process
session.referer_check     =         ; check HTTP Referer to invalidate 
                                    ; externally stored URLs containing ids
session.entropy_length    = 0       ; how many bytes to read from the file
session.entropy_file      =         ; specified here to create the session id
; session.entropy_length    = 16
; session.entropy_file      = /dev/urandom
session.cache_limiter     = nocache ; set to {nocache,private,public} to
                                    ; determine HTTP caching aspects
session.cache_expire      = 180     ; document expires after n minutes

[MSSQL]
;extension=php_mssql.dll
mssql.allow_persistent          =       On      ; allow or prevent persistent link
mssql.max_persistent            =       -1      ; maximum number of persistent links. -1 means no limit
mssql.max_links                         =       -1      ; maximum number of links (persistent+non persistent).  -1 means no limit
mssql.min_error_severity        =       10      ; minimum error severity to display
mssql.min_message_severity      =       10      ; minimum message severity to display
mssql.compatability_mode        = Off   ; compatability mode with old versions of PHP 3.0.

[Assertion]
;assert.active                          =       Off     ; assert(expr); does nothing by default
;assert.warning                         =       On      ; issue a PHP warning for each failed assertion.
;assert.bail                            =       Off     ; don't bail out by default.
;assert.callback                        =       0       ; user-function to be called if an assertion fails.
;assert.quiet_eval                      =       0       ; eval the expression with current error_reporting(). set to true if you want error_reporting(0) around the eval().

; Local Variables:
; tab-width: 4
; End:

; Zend optimizer
;zend_optimizer.optimization_level       =       7
;zend_extension                                          =       "/usr/local/Zend/ZendOptimizer.so"
---------------------------------------------------------------------------------

NB : I disabled Zend optimizer with version 4.0.2 (my optimizer beeing only working with version 4.0.1pl2)

Thank you anyway for your great work !
Greg.