In a new post to his blog David Dudok de Wit has a mini-tutorial on how to get you your PHP script connected with SharePoint to add new items to a list (based on an earlier post of his).

If you read my previous post (Reading a SharePoint list with PHP) you'll notice that the code is very similar. In fact, only the CAML query (which is contained in the SOAP request) and the Lists method has changed. Once again, I recommend following this simple guideline when coding a SharePoint application from PHP or Java.

He lists two resources that can help you get started on formatting the queries and some code that, using NuSOAP (yes, use can use PHP's SOAP extension too), connects to the remote SharePoint server and pushes across a SOAP message (XML) to update the given list's items.