Manual:$wgParserConf

Details
Associative array with the following members:

The class name
 * class :

The preprocessor class. Two classes are currently available: Preprocessor_Hash, which uses plain PHP arrays for temporary storage, and Preprocessor_DOM, which uses the DOM module for temporary storage. Preprocessor_DOM generally uses less memory; the speed of the two is roughly the same.
 * preprocessorClass :

If this parameter is not given, it uses Preprocessor_DOM if the DOM module is available, otherwise it uses Preprocessor_Hash.

The entire associative array will be passed through to the constructor as the first parameter.

Note that only Setup.php can use this variable -- the configuration will change at runtime via $wgParser member functions, so the contents of this variable will be out-of-date.

The variable can only be changed during LocalSettings.php, in particular, it can't be changed during an extension setup function.