PHP TutorialsPhp What is
PHP INI TutorialsPhp ini What is
The unserialize callback function will be called (with the undefined class name as parameter), if the unserializer finds an undefined class which should be instantiated.
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.
This directive allows you to control the response of the unserializer when a request is made to instantiate an undefined class.
For most users, this directive is irrelevant because PHP already outputs a warning in such instances if PHP's error reporting is tuned to the appropriate level.