example.php

<?php

require 'JSConcatinator.php';

// very basic usage example
header('Content-type: text/javascript; charset=UTF-8');

$concat = new JsConcatinator('javascripts');
echo $concat->concatinate('application.js');

JSConcatinator.php

<?php

class JsConcatinator {
	private $path;
	private $included;

	public function __construct($path){
		$this->path = rtrim($path, '/') . '/';
		$this->included = array();
	}

	public function concatinate($file){
		if (!$file = $this->pathToFile($file)) return '';

		$content = file_get_contents($file) . "\n";
		if (preg_match_all('/\/\/= require "(.*)"/', $content, $requirements)){
			foreach($requirements[0] as $key => $match){
				$content = str_replace($match, $this->concatinate($requirements[1][$key]), $content);
			}
		}

		return $content;
	}

	private function pathToFile($file){
		if (!$file) return false;

		$file = $this->path . $file;

		if (substr($file, -3) !== '.js'){
			$file .= '.js';
		}

		if (in_array($file, $this->included)){
			return false;
		}

		if (!file_exists($file)){
			throw new Exception("{$file} not found");
		}

		return $this->included[] = $file;
	}
}