prevent disclosing PHP version on PHP version check error

 * fixes #78
 * fixes sebsauvage#214

index.php

@@ -118,7 +118,7 @@ function checkphpversion()
     if (version_compare(PHP_VERSION, '5.1.0') < 0)
     {
         header('Content-Type: text/plain; charset=utf-8');
-        echo 'Your server supports PHP '.PHP_VERSION.'. Shaarli requires at least php 5.1.0, and thus cannot run. Sorry.';
+        echo 'Your PHP version is obsolete! Shaarli requires at least php 5.1.0, and thus cannot run. Sorry. Your PHP version has known security vulnerabilities and should be updated as soon as possible.';
         exit;
     }
 }