page1.php

<?php
session_start();
session_register("my_var");
$my_var="test";
echo $my_var;
?>

page2.php
<?php
session_start();
$a=session_is_registered("my_var");
echo "a has value $a";
echo $my_var;
?>

page1.php produces the expected output: test.
page2.php produces:
a has value

so the session is not working.

track_vars is enabled, register_globals is enabled.

php.ini file [sessions]

[Session]
session.save_handler = files        ; handler used to store/retrieve data
session.save_path = /phpsessions    ; 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          ; procentual 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.extern_referer_check =      ;
session.entropy_file =              ;
session.entropy_length = 0          ;              

the directory /phpsessions
is owned by the process that is running apache.

the directory contains files with names like this:
sess_7ec31a321f14710b8f6f96be239e0a2c

but they are empty (recorded as 0 bytes).

and the apache error log contains the following (which looks like it might have been output by php):

<b>Warning</b>:  write failed: Bad file descriptor (9) in <b>Unknown</b> on line
 <b>0</b><br>

<b>Warning</b>:  Failed to write session data. Please check that the current set
ting of session.save_path is correct (/phpsessions) in <b>Unknown</b> on line <b
>0</b><br>
<br>   

Apache version: 1.3.12
Debian Linux 2.2.13ac2 #1
loaded modules:
mod_php4, mod_setenvif, mod_unique_id, mod_expires,
                                    mod_auth, mod_access, mod_rewrite, mod_alias,
                                    mod_userdir, mod_cgi, mod_dir, mod_autoindex, mod_status,
                                    mod_negotiation, mod_mime, mod_log_config, mod_macro,
                                    mod_so, http_core

I have tried changing the session_save_handler to the same name as the owner of the apache process, but it makes no difference.  

Sessions were working just fine in the previous release of version 4 of PHP.