I have searched a little, and found SOAP and REST as web services protocols which relies on HTTP, both could be implemented with PHP.
there are many links in this page:
Code:
http://talks.php.net/index.php/Web+Services
for web services using PHP.
I do not know any thing before about web services, but it seems very interesting, but needs alot of reading.
consider this page too:
Code:
http://talks.php.net/show/xmlandphp5-phpcon-oct-2003/
Regards