Answer 1

Continuing on front-end's way you could go on jQuery.
Switching to back-end you can go on PHP first. But PHP philosphy is very different in compare with Javascript.

The next move more depend on do you want to be U.I developper? in this case you can go on jQuery to have fun, then Bootstrap.

Do you feel more interested in coding, without graphics, build complex codes, then go on back-end.

...or if you are like me => both cause both are amazing :')