PHP Find Ghostscript (cross-platform)

/**
 * Checks whether we may call gs through exec().
 *
 * @return bool|str If available, returns exe path. False otherwise.
 */
private static function getGhostscriptExecutable() {
   static $executable = null;

   if (is_null($executable)) {
      // we must be able to exec()
      $executable = self::isExecAvailable();
      if (!$executable) return $executable;

      // find on Windows system
      if ('WIN' === strtoupper(substr(PHP_OS, 0, 3))) {
         // look for environment variable
         $executable = getenv('GSC');
         if($executable) return $executable;

         // hope GS in the path
         $executable = exec('where gswin*c.exe');
         if(!empty($executable)) return $executable;

         // look directly in filesystem
         // 64- or 32-bit binary
         $executable = exec('dir /o:n/s/b "C:\Program Files\gs\*gswin*c.exe"');
         if (!empty($executable)) {
            return $executable;
         }

         // 32-bit binary on 64-bit OS
         $executable = exec('dir /o:n/s/b "C:\Program Files (x86)\gs\*gswin32c.exe"');
         $executable = empty($executable) ? false : $executable;
         return $executable;
      }

      // this is why I use Linux...
      $executable = exec('which gs');
      $executable = empty($executable) ? false : $executable;
      return $executable;
   }

   return $executable;
}

/**
 * Checks whether exec() may be used.
 * Source: http://stackoverflow.com/a/12980534/866618
 *
 * @return bool Whether exec() is available.
 */
private static function isExecAvailable() {
   static $available = null;

   if (is_null($available)) {
      $available = true;

      if (ini_get('safe_mode')) {
         $available = false;
      } else {
         $d = ini_get('disable_functions');
         $s = ini_get('suhosin.executor.func.blacklist');
         if ("$d$s") {
            $array = preg_split('/,\s*/', "$d,$s");
            if (in_array('exec', $array)) {
               $available = false;
            }
         }
      }
   }

   return $available;
}