My Function Composition implementation via PHP

compose.php

<?php

function compose(/* funs to compose */) {
    $fl = func_get_args();
    return function($x) use($fl) {
        $val = $x;
        while (null !== $f = array_pop($fl)) {
            $val = $f($val);
        };
        return $val;
    };
}

// ======================================================

$up = function($s) {return strtoupper($s);};
$y = function($s) {return $s.'!';};
$reverse = function($a) {return array_reverse($a);};
$head = function($a) {$v=array_values($a); return array_shift($v);};

$data = array('jumpkick', 'roundhouse', 'uppercut');

//$c1 = compose($y, $up, $head, $reverse);
$c1 = compose(compose($y, 'strtoupper'), compose($head, 'array_reverse'));
$c2 = compose(compose($y, 'strtoupper', $head), 'array_reverse');
$c3 = compose($y, 'strtoupper', $head, compose('array_reverse'));
$c4 = compose(compose($y), compose('strtoupper'), compose($head), compose('array_reverse'));

var_dump($c1($data));
var_dump($c2($data));
var_dump($c3($data));
var_dump($c4($data));

Doesn't the use of array_pop mean you can only use the resulting composed function once?