SQL is just for storing, sorting and querying data – it provides no ‘glue’ to turn that data into HTML… really that’s what PHP is best for – it’s rended horse gelatin in a jar, for sticking the various pieces together. Without a ‘controlling’ language like PHP, Perl or ASP, you cannot do a whole lot of useful stuff with just a SQL server.
Because there are MANY types of SQL server I suggest avoiding learning the mysql_ functions in PHP, which to be frank are outdated methodologies that really have no business being used on a page – while it involves a bit more learning the PDO object is generally more secure, more versatile – and it’s a GREAT introduction to object based programming for those who’ve never dealt with it before.
If you have a command of Javascript, then PHP should be a natural fit since it’s basically the same core language – part of the C syntax family. The only major differences that are likely to cause problems for you is the use of $ to indicate a variable, the use of a period to indicate string addition (as opposed to numeric additon), the lack of event driven processes, the different object model, and the VAST library of functions.
The last of those is really where PHP shines as an interpreted language. Interpreted – even bytecode interpreted – is SLOW… Userland code (the stuff you write) is significantly slower than system code (library functions compiled to native code), so PHP gives you a massive suite of functions, procedures, objects and methods… meaning you don’t have to re-invent the wheel for many simple tasks as dimes to dollars there’s already a procedure to do it, that would be many times faster than you could ever write it in PHP.
Which is why I read three pages of the php.net manual every night – I figure at that rate I’ve got another year and a half to go, and I’m constantly finding new and better ways of doing things in a language I’m already pretty good at.
Since you already know JS, I’d suggest digging straight into PHP.net’s manual and starting at the beginning – so you can familiarize yourself on the differences in variable handling, scope, etc.
http://php.net/manual/en/index.php
Oh, some simple suggestions that I find make for cleaner code… echo is king, double quotes suck, use commas to delimit echo output instead of string additions, and make use of the fact that single quote strings are white-space preserving in your output – and if you have to resort to more than one <?php ?> pairing in a file, you’re probably making life harder not only on yourself, but on the bytecode parser as well.