Example of overriding a "root level" function using a custom class. The example showcases how to override WordPress's pluggable.php function wp_mail, with a custom function.

function-override-with-class.test.php

<?php

class wpMandrill{
	static function load(){
		function wp_mail(){
			echo 'custom mail';
		}
	}
}

wpMandrill::load();


if(!function_exists('wp_mail')) {
	function wp_mail() {
		echo 'default mail';
	}
}

wp_mail();