How to pass a callback function as a parameter in PHP Back

In PHP, if you want to pass a callback function into another one, you are supposed to use call_user_func.

<?php
/** Definition */
function f($opt, $callback) {
    /** Operation */
    /** ... */
    call_user_func($callback);
}

/** Calling */
f($opt, function () use ($innerOpt) {
    echo 'callBack';
});
?>