Insert NULL in UNIQUEIDENTIFIER field

/*
How to insert a known UUID/GUID (record_id) - in this case generated by PHP, an unknown or blank UUID/GUID, and a regular int field

SQL Server Table creation code:

CREATE TABLE dbo.MYTable
    (
    [id] UNIQUEIDENTIFIER NOT NULL,
    [core_uuid] UNIQUEIDENTIFIER NULL,
    [category_id] UNIQUEIDENTIFIER NULL,
    [other_id] INT NOT NULL DEFAULT 0,
    PRIMARY KEY NONCLUSTERED (id)
    );

*/

// fix zero/null fields
$record_id = 'C39A4043-7DF9-4FCB-B5F5-C47F813A97B5';
$core_uuid = (!empty($core_uuid)) ? "'$core_uuid'" : "NULL";
$category_id = (!empty($category_id)) ? "'$category_id'" : "NULL";

// setup query  for MS SQL Server
$query = 'INSERT INTO [database].[schema].[table] ([id], [core_uuid], [category_id], [other_id]) '
  . 'VALUES (\''.$record_id.'\','.$core_uuid.','.$category_id.','.$other_id.')';

// execute query
$result = mssql_query($query);
if (!$result) {
    // query failed, print error to log
    $err = mssql_get_last_message();
    $log->lwrite('--> MSSQL error: '.$err);
}