Usually plugins will include wp-config.php or wp-load.php in order to gain access to core WordPress functions, but there are much better ways to do this.

It's best if you tie your processing functions (the ones that need but don't have access to core functions) into an action hook, such as "init" or "admin_init".

Please consult the Plugins API reference for more information: http://codex.wordpress.org/Plugin_API

If you're trying to use AJAX, please read this: http://codex.wordpress.org/AJAX_in_Plugins

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