diff --git a/UriSigner.php b/UriSigner.php
index 526a919738..66fba97266 100644
--- a/UriSigner.php
+++ b/UriSigner.php
@@ -75,7 +75,7 @@ public function check($uri)
         $hash = urlencode($params['_hash']);
         unset($params['_hash']);
 
-        return $this->computeHash($this->buildUrl($url, $params)) === $hash;
+        return hash_equals($this->computeHash($this->buildUrl($url, $params)), $hash);
     }
 
     private function computeHash($uri)
diff --git a/composer.json b/composer.json
index e082b2a32d..5526e718f1 100644
--- a/composer.json
+++ b/composer.json
@@ -21,6 +21,7 @@
         "symfony/http-foundation": "~2.7.36|~2.8.29|~3.1.6",
         "symfony/debug": "^2.6.2",
         "symfony/polyfill-ctype": "~1.8",
+        "symfony/polyfill-php56": "~1.8",
         "psr/log": "~1.0"
     },
     "require-dev": {