About learning Emacs
I recommend first of all using Ctrl-h t to do the Emacs tutorial. If you are still happy to carry on using Emacs after this initial experience, then I recommend you to buy a printed copy of the GNU Emacs Manual by Richard Stallman
Reading the whole of this book at the start of your Emacs learning process will save you much pain later on.
It's also much better than the O'Reilly book mentioned above.
What Emacs does for you
Emacs (I use cperl mode) can make writing source code much easier by doing things like indenting the lines of code automatically, or colouring in the key words and variable types. For example,
see this blog post, which was created using Emacs's colourings for cperl mode with htmlize.el.
Emacs for other languages
Emacs has patchy coverage for languages. Coverage for Emacs Lisp and C
is excellent, but it doesn't come with a JavaScript mode, although
there are ones that can be installed. The default CSS mode in
Emacs is pretty strange, but
it
is possible to set it up better.
Emacs' HTML mode seems OK to me, but I don't know anything about MySQL
(it has an SQL mode but I don't know anything about it), Python, or PHP.
As for mixed languages in the same file, as far as I know Emacs
doesn't do that at all well. You have to switch between modes in
different parts of the file.