How do you send NULL as a variable from PHP to SQL?

I want to send a value of "NULL" to a column in mySQL.

   $ParentEventID = "NULL";

   mysql_query("UPDATE events SET 
     ParentEventID = '$ParentEventID'");

The column keeps defaulting to "0". I have the column set to accept NULL and I can edit the cell with the NULL check box.

I need to not set the default to NULL because it may effect code in other places.