function my_big_function()
    {
    	static $result;
    
    	// Function has already run
    	if ($result !== NULL)
    		return $result;
    
    	// Lot of work to determine $result
    
    	$result = TRUE;
    	return $result;
    }

Also see: http://stackoverflow.com/questions/6188994/static-keyword-inside-function