fsockopen question

I am looking to process credit card transactions on my site. In order to do this, I need to open a socket to a particular URL. I then submit an XML string to them. They process the string and send me a response. I am going to a certain (secure) location on the site of my credit card processor (https://www.myprovider.com/XMLSubmit/processtrans.asp) and then submit the XML string. After I submit all of the XML, the processor will give me a response as to whether or not the transaction is approved. How do I send the XML string to the above URL? When I use fsockopen, it doesn't like anything other than an IP address or a host name (ie www.myprovider.com). I don't even really see where to force this to go over HTTPS. I set the port to 443, but need to be sure that the data going across uses SSL. How do I send this XML string over HTTPS to https://www.myprovider.com/XMLSubmit/processtrans.asp?