php - check for valid guid

I have a mssql database in which my primary keys are GUIDs. I am working on a web interface for inserting some data. I need a GUID, generated by php. I am using com_create_guid() function. So, before trying to insert I want to ensure that my parameters are valid. I can not find a way to check if a string(com_create_guid() returns string) is a valid GUID.