This script, generates an error in the php log file

session_start();

The error says:

PHP Warning: Failed to write session data. Please check that the current setting of session.save_path is correct (/tmp/) in Unknown on line 0 

The same code with the same version of apache/php works fine on Linux.  I simply copied the code over.

Here's the session section in php.ini; it's the same on both machines.

[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 

Here's my compile options:

./configure
--with-apache=/usr/local/src/apache_1.3.12
--with-mysql
--enable-track-vars
--with-config-file-path=/etc
--enable-trans-sid