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Installing/Configuring
PHP Manual

## Runtime Configuration

The behaviour of these functions is affected by settings in php.ini.

Variables Configuration Options
Name | Default | Changeable | Changelog
--- | --- | --- | ---
unserialize_callback_func | NULL | PHP_INI_ALL | Available since PHP 4.2.0.

For further details and definitions of the PHP_INI_* modes, see the Where a configuration setting may be set.

Here's a short explanation of the configuration directives.

`unserialize_callback_func` string

  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.

  See also unserialize and Autoloading Objects.