When sending reservation id's with PHP sometimes an error is received, for example when calling "cancelReservation", while other functions are operating correctly.
Problem is with 32 bit limitation of your PHP server and large integer numbers will overflow because of it.
The obvious solution is to move to 64 bit server, but if that is impossible for you, the most common solution when there is a problem with native PHP SOAP component is to build your SOAP envelope manually and send it over to the web service via simple POST operation.
That is one approach that you can use to solve your problem with methods that require long id's. Please consult: