the essentials
What PHP actually is.
Written for someone deciding, not for someone who already knows. Skip it if you do.
What PHP is
A programming language made for building websites, and one of the few that has done nothing else for thirty years. When you open a page, PHP is the part that runs on the server: it reads the database, works out what you should see, and hands the browser finished HTML. You never see it and you never install anything. Roughly three quarters of the websites whose language can be identified run on it, WordPress, Wikipedia and a large share of online shops among them.
Why so much of the web runs on it
It's cheap to host, it runs on essentially any server, and it has thirty years of solved problems behind it: payments, invoices, VAT, shipping, PDFs, e-mail. For most business software the hard part isn't the language, it's the hundred small things a real system needs, and in PHP most of those already exist and have been tested by a very large number of people. That's also why finding somebody to take over a PHP project later is easier than for almost anything else.
Where it fits, and where it doesn't
It fits catalogues, shops, booking systems, portals, invoicing, internal tools and anything with a database behind a login. It's not the right answer for something that has to hold thousands of live connections at once, like a chat or a trading screen, and it's not what you would reach for to build a mobile app. We'll say so when that's the case rather than sell you the thing we happen to be on.
The version question, which is the real one
PHP has changed enormously and most of its reputation comes from versions nobody should still be running. Current PHP is several times faster than PHP 5 and stricter about the mistakes that used to cause security problems. A version past its end of life gets no security patches at all, so known holes simply stay open. If you're on one, moving off it's usually the highest-value work available to you, and it's rarely as bad as feared.
Frameworks, and working without one
Most new PHP work sits on Laravel or Symfony, which supply the routing, the database layer, the queues and the security defaults so nobody rebuilds them badly. Plenty of what we inherit uses no framework at all, which is normal for older systems and not a reason to rewrite: we work in what is there, add tests around the parts that earn money, and introduce structure only where it pays for itself.
Things you already use that run on it
WordPress, which is a large share of the web on its own, along with Wikipedia, Slack’s original back end, Mailchimp and a great many banking and government systems that nobody advertises. It's not a language that quietly disappeared; it's one that stopped being talked about because it works.