For information on how variables behave, see the Variables entry in the Language Reference section of the manual.
The behaviour of these functions is affected by settings in php.ini.
For further details and definition of the PHP_INI_* constants see ini_set().
Here's a short explanation of the configuration directives.
The unserialize callback function will called (with the undefind class' name as parameter), if the unserializer finds an undefined class which should be instanciated. A warning appears if the specified function is not defined, or if the function doesn't include/implement the missing class. So only set this entry, if you really want to implement such a callback-function.
See also unserialize().