Here's a short explanation of
the configuration directives.
unserialize_callback_funcstring
The callback specified is called when unserialize
attempts to use an undefined class. A warning appears if the specified
callback function is not defined, or if the callback function fails to
define the missing class.
The maximum depth of structures permitted during unserialization when
using unserialize, and is intended to prevent stack
overflows. This can be disabled by setting
unserialize_max_depth=0.