This is Aphyrs "Reversing the technical interview" in PHP.

reversing_the_technical_interview.php

<?php

// This is Aphyrs "Reversing the technical interview" in PHP.
// https://aphyr.com/posts/340-reversing-the-technical-interview

function cons($a, $b) {
    return function ($x) use ($a, $b) {
        return $x ? $a : $b;
    };
}


function nth($n, $l) {
    if ($n == 0) {
        return $l(true);
    }
    if (is_null($l)) {
        return null;
    }
    return nth($n-1, $l(false));
}

function pprint($l) {
    echo "[";
    while($l) {
        echo $l(true);
        $l = $l(false);
        if (!is_null($l)) {
            echo ", ";
        }
    }
    echo "]";
}

function reverse($l, $r = null) {
    if (is_null($l)) {
        return $r;
    }
    return reverse($l(false), cons($l(true), $r));
}

pprint(reverse(cons(1, cons(2, cons(3, null)))));