We can use either session_register or $_SESSION for registering session variable in php.
Find below the difference between both of them.
session_register() is a function and $_SESSION is a superglobal array.
session_register() is used to register a session variable and it works only when register_globals is turned on. (Turning ON register_globals will create mesh-ups, but some applications (e.g OScommerce) requires turning ON of register_globals)
But $_SESSION works even when register_globals is turned off.
If session_start() was not called before session_register() is called, an implicit call to session_start() with no parameters will be made. But $_SESSION requires session_start() before use.
session_register function returns boolean value and $_SESSION returns string value
session_register() function has been DEPRECATED as of PHP 5.3.0 and REMOVED as of PHP 6.0.0.
If you are not familiar with session, you can read my previous post.