You could use the HTTP protocol to comunicate with the PHP Server .
In c# this communication can be achieved using the HttpWebRequest Class.
Encapsulating the data you wish to send via POST or GET methods
HttpWebRequest Class
http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx#Y0
HttpWebRequest POST
http://www.netomatix.com/httppostdata.aspx
HttpWebRequest GET
How do I use HttpWebRequest with GET method
On the PHP Side you would extract the parameters you transferred.
The method of parameter extraction will depend on weather you used POST or GET to send data to the server.
PHP GET
http://php.net/manual/en/reserved.variables.get.php
PHP POST
http://www.w3schools.com/php/php_post.asp
And once you got your parameters on PHP server side you proceed doing something with them.
Read more about PHP in general
http://php.net/manual/en/index.php
And about the HTTP Protocol.
http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol
good luck