doctrine2/manual/docs/Getting started - Compiling.php

Compiling is a method for making a single file of most used doctrine runtime components including the compiled file instead of multiple files (in worst cases dozens of files) can improve performance by an order of magnitude. In cases where this might fail, a Doctrine_Exception is throw detailing the error.

<code type='php'>
Doctrine::compile();
// on some other script:
require_once('path_to_doctrine/Doctrine.compiled.php');
</code>