Как в одной функции вызвать другую php?

<?php

function firstFunction()
{
    echo "This is the first function | ";
}

function secondFunction()
{
    echo "This is the second function";
}

function finalFunction()
{
    call_user_func('firstFunction');
    call_user_func('secondFunction');
}

echo finalFunction(); // => This is the first function | This is the second function