A semi-workable implementation of functional composition in PHP.

compose.php

<?php
/*
This function takes in a bunch of callables (closures, functions, strings naming functions, etc.)
and returns a function such that compose(f, g)(x) <=> f(g(x)). In other words, the return function
will take exactly one argument, and call the functions supplied to compose right to left
on that argument.
*/
function compose() {
    $functions = func_get_args();
    return (function() use ($functions) {
        $intermediate = func_get_arg(0);
        for ($i = (count($functions) - 1); $i >= 0; $i--) {
            $fn = $functions[$i];
            $intermediate = $fn($intermediate);
        }
        return $intermediate;
    });
}

// Proof of concept.
function f($x) { return $x * $x; }
function g($x) { return 2 * $x + 4; }
$fg = compose('f', 'g');

echo f(g(5)) . " " . $fg(5);