AssetExistsExtension.php

<?php
/**
 * Twig Filter for Symfony 2 - check if asset exists.
 * 
 * Modified Alain's Tiemblo code: http://stackoverflow.com/a/16906315/2512304
 * 
 * Installation:
 *   1. Add this file to your bundle into src/Acme/DemoBundle/Twig/AssetExistsExtension.php
 *   2. Update namespace, if your bundle isn't Acme/DemoBundle
 *   3. Register it in app/config.yml:
 *      services:
 *          assetexists.twig.extension:
 *              class: Acme\DemoBundle\Twig\AssetExistsExtension
 *              arguments: ['@kernel']
 *              tags:
 *                  - { name: twig.extension }
 *     If you're using different web directiory, you should specify it as the second argument, e.g.:
 *              arguments: ['@kernel', 'public_html']
 * 
 * Usage:
 *   {% if asset_exists('images/my_picture.jpg') %}
 */

namespace My\FrontendBundle\Twig;

use Symfony\Component\HttpKernel\KernelInterface;

class AssetExistsExtension extends \Twig_Extension {
	private $kernel;
	private $webDir;
	
    public function __construct(KernelInterface $kernel, $webDir = 'web') {
        $this->kernel = $kernel;
        $this->webDir = $webDir;
    }
	
	public function getName() {
		return 'assetexists';
	}

	public function getFunctions() {
		return array('asset_exists' => new \Twig_Function_Method($this, 'asset_exists'));
	}

	public function asset_exists($path) {
		$webRoot = realpath($this->kernel->getRootDir() . '/../' . $this->webDir);
		$toCheck = realpath($webRoot . '/' . $path);
		
		if (!is_file($toCheck))
			return false;
		
		// check if file is well contained in web/ directory (prevents ../ in paths)
        if (strncmp($webRoot, $toCheck, strlen($webRoot)) !== 0)
			return false;
		
		return true;
	}
}