Как сделать вызов функции из другой функции php?

<?php

function firstFunction() {
    echo 'This is the first function!';
}
 
function secondFunction() {
    firstFunction();
}
 
secondFunction(); // => This is the first function!

call_user_func(callable $callback, mixed ...$args): mixed

<?php

function firstFunction() {
    echo 'This is the first function!';
}
 
function secondFunction() {
    call_user_func('firstFunction');
}
 
secondFunction(); // => This is the first function!