I believe that what you are trying to accomplish would be done with the following:
However, when I do this I get the following Exception:
The request failed schema validation: The element 'http://schemas.microsoft.com/exchange/services/2006/types:SearchExpression' is abstract or its type is abstract.
The code above should create the proper XML according to the XSD, but it appears that PHP has trouble forming the XML due to the use of abstract types within the XSD. If anybody has a solution to this I would love to hear it.