PHP で関数合成 ref: http://qiita.com/yuya_takeyama/items/858c5a0652441f54f0a8

compose.php

<?php
class ComposableFunction
{
    /**
     * @var callable
     */
    private $fn;

    /**
     * Constructor
     *
     * @param callable $fn
     */
    public function __construct(callable $fn)
    {
        $this->fn = $fn;
    }

    /**
     * Invokes function
     */
    public function __invoke()
    {
        return call_user_func_array($this->fn, func_get_args());
    }

    /**
     * Composes new function
     */
    public function compose(callable $fn)
    {
        return new static(function () use ($fn) {
            return $fn(call_user_func_array($this, func_get_args()));
        });
    }
}

$splitWithUnderscore = new ComposableFunction(function ($str) {
    return explode('_', $str);
});

$mapUcfirst = function ($words) {
    return array_map('ucfirst', $words);
};

$camelize = $splitWithUnderscore->compose($mapUcfirst)->compose('join');

var_dump($camelize('foo_bar_baz'));
// => FooBarBaz