Netbeans shows a strange behavior with the 'uninitialized variable' hint:

/*public static*/ function registerShutdownFunction($callable) {
   while(isset(self::$shutdownFunctions[$id = uniqid()]));
   self::$shutdownFunctions[$id] = $callable;
   return $id;
}

The variable $id is definitely assigned after the while statement. So in the nextline, Netbeans does not show any hint regarding this variable (correct). But in the return line, $id is reported as possible uninitialized.
This behaviour only occurs when the three lines are wrapped in a function (normal or class method) but not when used in global scope.