code: public function searchFullPath($filename) {
          $paths = explode(PATH_SEPARATOR, get_include_path());
          foreach ($paths as $path) {
              $fullPath = $path . DIRECTORY_SEPARATOR . $filename;
              if (file_exists($fullPath)) {
                  return $fullPath;
              }
          }
          return false;
      }
docs: Find a filename in the include path fallback
  
  @param $filename
  @return bool|string