apply_filter_hook.php

<?php
/* 
custom filter hook
*/

function custom_filter_something_cool() {
 
    $output = 'Hello World';
 
    return apply_filters('my_custom_filter', $output);
}

echo custom_filter_something_cool();


//Place in functions.php or else
function custom_filter_function( $text ) {
    return 'The updated text will appear.';
}

add_filter('my_custom_filter', 'custom_filter_function');