PHP is relevant and would be for a very long time, more than 50% of websites globally are ran on php. With major cms like wordpress, joomla etc built on php, its not about to go easily.
It is also very easy to get jobs as a freelancer if you know PHP being that a lot of demands exists for it, may not be high level jobs but its there and in so much demand.
If you are willing to learn PHP you can refer this:PHP Tutorial | Beginner's Guide to PHP - Edureka
Hope this is helpful!