What language should I learn next (after HTML & CSS) ? Should it be JavaScript or PHP ?

It depends on where You want to go. If you want to head towards Backend development then PHP, if you want to continue mastering the Frontend development then JS is the way to go. That is completely up to you on what YOU want as a developer. Eventually ofcourse its good to know both.

If you're on track to become a fluent front-end designer/developer, you'll definitely want a grip on JS. PHP is one of the shorter tracks on Treehouse, so you could run through it and come back to JS. Another idea would be to wait a bit before picking up another language and, in the meanwhile, work on some sites showcasing your HTML and CSS -- rock some animations! -- then come back to it with a fresh mind. You'll know what's best -- GL!

I would say if you have done html and css, you are learning a lot of front end stuff, you should complete your front end knowledge by learning javascript and then move onto server sided stuff. I think it's a good idea to do both, but javascript maybe first. This way you have a full arsenal of skills before moving onto the server sided stuff which I think would be the next step. I would also say it's important to make sure that you absolutely have a good grasp of HTML. CSS and Javascript, perhaps by building multiple projects to showcase your skill until you move onto the server side of things.

PHP and Javascript are quite different, not just in the way they work but in what they do. You can do a lot of things and build a lot of websites without PHP (or any server sided language) and if you have good enough skills plus can showcase these you can also find employment as a front end web developer/designer, but usually javascript is something you will also be required to know in this area, if you skip ahead and do PHP, you are leaving this area of your skill set out and will have to come back to it later. You mentioned that you found javascript difficult previously, I wouldn't say personally that learning PHP will make javascript learning any easier, so I would recommend just jumping in and mastering this first, as you will most likely be required to do it at some stage.

In short, my view is that it is best to finish one area before moving onto the next and the next logical step for you is Javascript, I am sure you believe this too, but perhaps your resistance towards it, is due to you trying it before. Doing this will round off your front end development skills before you move onto the next step with is server sided skills (which you may also want to explore other languages than just PHP)