Don't ask why

cons.php

<?php

/**
 * cons for PHP
 */

// constructs a pair
function cons($a, $b) {
   return function($m) use ($a, $b) {
      if ($m == 0) {
         return $a;
      } else if ($m == 1) {
         return $b;
      } else {
         throw new Exception("m is not 0 or 1 CONS");
      }
   };
}

// returns the first item in a pair
function car($x) {
   return $x(0);
}

// returns the second item in a pair
function cdr($x) {
   return $x(1);
}

// same as car, mostly used on lists
function first($x) {
   return car($x);
}

// same as cdr, mostly used on lists
function rest($x) {
   return cdr($x);
}

// constructs a list built from nested pairs
// (list 1 2 3 4)
function plist(...$items) {
   if (empty($items))
      return null;

   return cons(array_shift($items), plist(...$items));
}

/**
 * Example Code
 */

$pair1 = cons(1, 2);
$pair2 = cons(3, 4);

// "1"
car($pair1);
// "2"
cdr($pair1);

// nested pair
$pair = cons($pair1, $pair2);

// "1"
$one = car(car($pair));
// "3"
$two = car(cdr($pair));

// Recursive sum example
function sum($list) {
   if (is_null($list))
      return 0;

   return first($list) + sum(rest($list));
}

// "10"
sum(plist(1, 2, 3, 4));