Description:
------------
PHP warnings are issued as
PHP Warning:  session_start(): Unknown session.serialize_handler. Failed todecode session object. in - on line 2
PHP Warning:  Unknown: Unknown session.serialize_handler. Failed to encode session object. in Unknown on line 0

These happen because session module try's to find serialize_handler before
serializer module is loaded (and registered itself as valid serialize_handler)

There are check for save_handler (PS(mod) != NULL) in the
PHP_RINIT_FUNCTION(session), but not for PS(serializer)

I wrote a short patch http://phadej.daug.net/php-5.2.6-serialize_handler.patch
against php-5.2.6. PHP_RINIT_FUNCTION seems to be same in php-5.2 and php-5.3
brances.


Reproduce code:
---------------
php -d "extension=wddx.so" -d 'session.serialize_handler=wddx' <<\EOF
<?php
        var_dump(session_start());
?>
EOF

Expected result:
----------------
bool(true)

Actual result:
--------------
PHP Warning:  session_start(): Unknown session.serialize_handler. Failed to decode session object. in /home/phadej/dev/php-sess-bug/- on line 2
bool(true)
PHP Warning:  Unknown: Unknown session.serialize_handler. Failed to encode session object. in Unknown on line 0