From PhpCOIN Documentation
While phpCOIN has many features and functions built-in, sometimes you need to add a function for yourself.
phpCOIN accomodates this through the /coin_includes/custom.php file. In this file, add whatever php functions you want or need ~ phpCOIN automatically includes this file, so the functions will be available throughout phpCOIN.
The disadvantage of the /coin_includes/custom.php file is the same as the language files ~ phpCOIN upgrades will over-write your customizations. The solution is the same ~ create a file called /coin_overrides/custom_override.php and place your custom functions within it. When phpCOIN is upgraded, it will not touch /coin_includes/custom_override.php, so your customizations will not be lost.