Just to throw in my two cents, I actually do PHP development for a living, every day, in a real brick-and-mortar shop (as I'm sure many do, but this is important).
First, it is absolutely fundamental to have a good HTML/CSS/JavaScript foundation that should include an understanding of browser differences, box model, AJAX from the client side (And therefore the concurrency-with-a-twist that is asynchronous web requests), and Document Object Model. Of course, there's no reason you can't learn all this concurrently as your learn PHP, but you must learn it or you will suck.
Now, if your C/C++ background is strong, PHP will be an absolute cake walk -- there will be almost no learning curve (syntactically; semantics is another deal, of course), you will recognize where PHP's weaknesses are, and recognize that they are mostly technique-based; being smart, keeping your head about you, and keeping your goals first and foremost will help. Do a bit of research on the history of PHP and why register_globals
was a horrible idea, so you don't repeat the mistake and write something like this:
foreach ($_POST as $pkey => $pval)
$$pkey = $pval; // DO NOT EVER DO THIS FOR THE LOVE OF ALL THAT IS HOLY
If you write good code, it will show no matter what language you use (except for original BASIC, of course). Always remember, to avoid bad habits, pretend that the next person who will maintain your code is a psychotic, murderous PHP wizard who knows where you live. This will keep you using BCP and not just copy/pasting the same horrible code everywhere, which will in turn make your own life (maintenance-wise) much more productive and enjoyable.
Finally, please ignore the massive naming inconsistencies, there are reasons for this that will become clear if you read up on PHP history (esp. versions 1-3); they may not be good reasons, but they are reasons. Also, Unicode can be a big "Gotcha!" when writing PHP web apps -- if there is any chance anything other than good old single-byte ASCII-compatible characters will be coming through, double-and-triple-reinforce your app for this.