In sense with "PHP-universe" PHP support for any advanced SOAP sucks big time. You will end up using something like http://wso2.com/products/web-services-framework/php/ as soon as you cross the basic needs, even to enable WS-Security or WS-RM no inbuilt support.
SOAP envelope creation I feel is lot messy in PHP, the way it creates namespaces, xsd:nil, xsd:anytype and old styled soap Services which use SOAP Encoding (God knows how's that different) with in SOAP messages.
Avoid all this mess by sticking to REST, REST is nothing really big we have been using it since the start of WWW. We realized only when this http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm paper came out it shows how can we use HTTP capabilities to implement RESTFul Services. HTTP is inherently REST, that doesn't mean just using HTTP makes your services RESTFul.
SOAP neglects the core capabilities of HTTP and considers HTTP just as an transport protocol, hence it is transport protocol independent in theory (in practical it's not the case have you heard of SOAP Action header? if not google it now!).
With JSON adaption increasing and HTML5 with javascript maturing REST with JSON has become the most common way of dealing with services. JSON Schema has also been defined can be used for enterprise level solutions (still in early stages) along with WADL if needed.
PHP support for REST and JSON is definitely better than existing inbuilt SOAP support it has.
Adding few more BUZZ words here SOA, WOA, ROA
http://blog.dhananjaynene.com/2009/06/rest-soa-woa-or-roa/
http://www.scribd.com/doc/15657444/REST-White-Paper
by the way I do love SOAP especially for the WS-Security spec, this is one good spec and if someone thinking in Enterprise JSON adaption definetly need to come with some thing similar for JSON, like field level encryption etc.
