The problem is that the session_register is _stronger_ than the post method. 
When I start a session and register a variable and give a value
to this variable, then the form post will not override the original value.

Solaris 2.6/2.7 Apache/mod_php
php.ini:
[Session]
session.save_handler = files        
session.save_path = /tmp                                            
session.name = PHPSESSID           
session.auto_start = 0           
session.lifetime = 0              
session.serialize_handler = php     
session.gc_probability = 1       
session.gc_maxlifetime = 1440