Core PHP does not include a define_once() function to complement functions like require_once() and include_once(), which is pretty silly in my opinion. While I am generally not a fan of using *_once statements due to the performance penalty (and incurred laziness), define_once is the exception. There are ways to look for a loaded/missing file, but a define is not a define until you define it, so you really have no choice. So in situations where you have to blindly load defines — I do it to build language defines in a cascading templating system — use this function to achieve the proper results:

function define_once($define, $value) {

  if (!defined((string)$define)) {

    define($define, $value);

    return true;

  }

  return false;

}

```