First-class functions in PHP.

firstclass.php

<?php
/* 
 * A demonstration of first-class functions in PHP.
 * Blog post: http://www.sbisbee.com/blog.php?id=2352301864
 *
 * By Sam Bisbee <www.sbisbee.com> on 2010-12-17
 */

function makeLouder($fn)
{
  return function($a) use($fn) {
    return $fn($a)."!!!";
  };
}

$foo = makeLouder(abs);
echo "<p>".$foo(900)."<br/>".$foo(-900);

$bar = makeLouder(strtoupper);
echo "<p>".$bar("upper");

function makeDerivative($fn, $deltaX)
{
  return function($x) use ($fn, $deltaX) {
    return ($fn($x + $deltaX) - $fn($x)) / $deltaX;
  };
}

$cos = makeDerivative(sin, 0.00000001);
echo "<p>".$cos(0);        // 1
echo "<p>".$cos(pi() / 2); // 0
?>