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PHP isn't dead
So the following are the top reasons why you should probably learn PHP. Note that I do not know PHP at a professional level. However, from my own professional work experience I can safely say that PHP is everywhere. At my previous employer, which managed over 200 websites, at least 90% of the websites were in PHP and the company only had 1 full time .NET developer. You can guess who that one is.
It's incredibly stable
PHP has been around for some time now. It began around 1995, which makes it over 2 decades old now. Which means that it's a stable language. You won't get any weird bugs anymore. At least you probably won't. More than likely you won't.
However, the newer languages and frameworks are still some years away from getting to those levels. Just taking a look at React as an example, we can see that there are still a fair number of issues that need to be resolved currently.
And again, that's not a negative thing. The only way that React can get to a stable place is to get its bugs reported, listed and then fixed. And in another decade or so it too will have a slightly higher market share and a more stable code base.
It is incredibly well documented
If there's anything that you wish to do that involves HTTP in some way, then not only can PHP handle it, but somebody has already handled it, and more than likely written about it online. And that goes in hand with it's massive market share, which we'll touch upon down below. But it also has alot to do with its fantastically robust documentation. Official documentation one should add.
If you're just getting started with PHP or if you're a seasoned vet, the official PHP manual has something for you to learn in some shape way or form. This is a big part of what makes PHP such a wide spread and easily digestible language.
Wordpress uses it
Sites like Wix and Squarespace are quickly gathering a good chunk of the newer market share. However, that is still only a small fraction. More than likely people that sign up for the free tiers on these website templating sites don't stick around too long. Wordpress however, is a different story. It actually requires developers for the most part. And it is written in, you guessed it, PHP.
You can implement Wordpress without knowing PHP sure. But you can not build on top of or modify Wordpress without knowing PHP. Plugins and widgets all require knowledge of the language. Wordpress accounts for a good percentage of total CMS based websites currently, which again, automatically puts PHP on that level as well. So by knowing PHP, you get to be a part of the ever popular Wordpress group. This not only makes you more knowledgeable around more websites, but increases your access to potential paid work.
Most websites in the world use PHP
The best reason for learning PHP is that if you want to get a job in the programming world, you're more than likely going to need to. It's okay if someone learning to code is using Python and posting it on social media networks to a rousing team of support. In fact, it's nice to see that kind of encouragement these days. However, the fact of the matter is that most websites currently in existence rely on PHP in some shape, way or form. According to w3techs statistics, a whopping 82.9% of all reported websites are currently using PHP.
This of course depends on what you are learning to code for. If it's to grow your skills in the professional world, then it is definitely good to know it and know it well. If it's to work on something on the side as a hobby, then you probably should go for one of the newer and more trendy languages, as you can ride with the support from all the new adopters to that language.
At the end of the day, a programming language is a tool that you decide to acquire and to strengthen in order to help you achieve something. There's no better or worse. React isn't better than PHP because it's talked about more. In fact, that's probably detrimental. If anyone ever says that a language is superior or inferior, then one should be wary about that persons skillset and their experience level. If you're amazing React developer, then congratulations. Own that skill and make it work for you. And if you're a seasoned PHP dev that's worked for Fortune 500 companies, then the same goes out.