Custom

A feature is called ‘custom’ when it is not part of the standard installation of PHP, any PHP extension or a publish library. For example, the function foo() is generally a custom function.

Any declaration object may be custom : constants, variables, classes, interfaces, enums, traits.

A custom asset is usually the last to be defined, and, as such, has to use a distinct name. For example, declaring a function called wp_header is possible as long as the code is not part of Wordpress, which does declare it as a common set of features.

<?php

// This is a custom function
function foo() {}

?>

Related : Native