lanstein has submitted a two-part article series looking at connecting your PHP application with the Salesforce.com APIs (as a part of a blog dedicated entirely to the subject):

PHP and Salesforce is a new blog dedicated to helping web developers take control over their CRM integration. Most of the early entries will focus on building a reference of Salesforce PHP examples, followed by discussions on topics including performance, concurrency, CRM framework architecture (and whether you should consider building one), and strategies for handling scheduled and unscheduled outages.

The first article gives an overview of how to make the connection, where to get the tools you'll need and provides a bit of sample code to let you test out a connection. The second article looks at lead generation in the Salesforce application.

If you're interested in the Salesforce+PHP combination, you should definitely check out this blog.