If you want to use the web-to-lead functionality, you might also consider the (arguably) easier method of posting back and then submitting the lead from your script. Generally, it looks like this:

1) Check to see if you received a "POST".
2) If not, display the form.
3) If so, check to ensure all field values are valid.
4) If not, display the form with errors.
5) If valid, construct a HTTP Post request into a string.
6) Open a socket to www.salesforce.com.
7) Send the data through the socket.
8) Check response for "IsProcssed: True" (this lets you know you can close the socket safely)
9) Display thank you page.

I recently did this for a project in PHP, in fact, and it's actually pretty easy to accomplish.

You would want to use this method if: a) you don't have the API available, b) you don't want to try and install the library, c) you don't want to take the time to learn the library, or d) you don't want to necessarily wait for an API response (usually 2-3 times longer than simply posting to Web to Lead because of the processing overhead, especially if you're not caching login sessions).

You would use the API though, if you wanted to implement: a) Salesforce-controlled validation, b) guaranteed submissions to the server (technically, Web-to-Lead is guaranteed, but only if you have the correct oid parameter), c) other logic that might benefit from this method, such as offering specials that are loaded from the server based on some logic in Salesforce, or offering a chance to modify the data, etc.