<?php
function myfunction () {
    echo 'hello world<br>';
}

call_user_func('myfunction');

function myfunction1 ($arg) {
    echo 'hello world' . $arg . '<br>';
}

call_user_func('myfunction1', '2018');

class Myclass {
    public function testUserFunc() {
        // you can call a function from outside your class
        call_user_func('myfunction');
    }
}

$myclass = new Myclass;
$myclass->testUserFunc();