What's the difference between a 500 Internal Server Error and a 404 Not Found error?

A 500 error indicates a problem *within* the server itself. It usually means the PHP code encountered an error. A 404 error, on the other hand, indicates that the requested resource (e.g., a webpage) could not be found on the server. The 500 error points to a problem in the code; the 404 error points to a problem with the requested URL. They are distinct types of HTTP errors.