这个快速排序功能有什么问题？

function quick($a) {
    if (count($a) < 2) return $a;
    $l = [];
    $r = [];
    $pivot = $a[0];
    foreach ($a as $val) {
        if ($val > $pivot) {
            $r[] = $val;
        } else {
            $l[] = $val;
        }
    }
    return array_merge(quick($l), [$pivot], quick($r));
}

print_r(quick($a));

I get this error:

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 72 bytes) in /Applications/XAMPP/xamppfiles/htdocs/sort.php on line 46

Line 46 is $l[] = $val;

Quite simple. You're getting nearly infinite recursion.

The reason is that you're not excluding the pivot point from the sub-arrays. So $l will always contain it. And if $pivot is not the smallest value in the array, you'll infinitely recurse with an empty $r array, and copying $a into $l...

Instead, you need to adjust your if condition and look at the pivot key:

$pivot = $a[0];
foreach ($a as $key => $val) {
    if ($key === 0) {
        continue; // pivot
    } elseif ($val > $pivot) {
        $r[] = $val;
    } else {
        $l[] = $val;
    }
}