This file can be used to add arbitrary PHP code to be run when a page loads. You might use this to change some of the configuration constants defined in blogconfig.php or to call ini_set() to change the PHP configuration. This file does not exist in the default install and must be created if you want to use it.
This is a regular PHP file. Because it is called when a page first loads, it is important that all code be enclosed in PHP tags and that there be no characters or whitespace outside the PHP tags. If there is, then you will get errors. Likewise, any PHP syntax errors in the file will generate errors when the file loads. Therefore, this is recommended only for people who know what they’re doing.