Very, very shallow learning curve. Unlike some languages, you don't need to become near-fluent before you can use it for anything at all. You can learn three words of PHP, just enough to do one thing, and then add other words as you need them.
If you are looking to add interactive (with your web audience) php is probably the best place to start. There are other languages such as perl which is also great (in my opinion, the best) for web programming but finding answers for php problems is probably the easiest. There are sites like [html.net ] which along with [w3schools.com ] which can get you off the ground quick at no expense, and, of course you are already at the best place of all.