On DZone.com there's a recent post from Giorgio Sironi listing out some of the HTTP client utilities that can be used in your PHP applications.

In the PHP world, HTTP is very important: not only for receiving requests, which is the job of PHP application, but also for performing them. ROT web services are probably the most popular way to interface with external systems nowadays. We'll make no assumptions on the payload of HTTP requests (which may be binary, text, JSON, XML), so you'll have to deal with that yourself. In some cases, you may choose a more specific solution such as the SoapClient class in the SOAP extension, or the XML-RPC extension.

Among the methods he mentions are things like the cURL extension, the PECL HTTP extension. He also talks about some of the libraries out there that are popular among developers (Buzz and Guzzle) as well as the modules included in the Zend Framework (v1 and v2) to make the requests.