PHP manual - your friend
One of the things that makes coding PHP such a hassle-free experience is the excellent manual over at php.net. It's very easy to find what you're looking for, there are a lot of examples and there are also user-contributed notes, which add even more value to the manual, because people post solutions they have found to common or not so common problems.
Easy URLs
You can go to php.net and either browse or search for the function you're currently looking for, but you can also search directly from the address bar of the browser. Just type http://php.net/date and you'll end up on the help page for the date() function. It's OK if you don't know the exact name of the function, the site will try to make the best of your query. For example if you mistype in the address bar http://php.net/includ (forget the "e" in "include") the site will still load the page for the include statement. If the query is not so obviously mistyped, you'll get a list of suggestions. This is also one way to "discover" PHP functions, when you have a feeling that a certain function probably exists, but you don't know the exact name.
Source code
Another pretty cool thing about the php.net site is that you can see its source code, right from the pages itself. Just visit any page on php.net, then scroll down to the very bottom and look for the link called "show source". It will give you the code of the page you're looking at. Don't be scared if you don't quite understand what's going on the page, soon you'll learn to make sense of it all.
Try it yourself
Take some time to look around and get familiar with the php.net site, for example look up the functions and statements we've seen so far - date(), include, require, echo, ini_set().