PHP is the most popular language but it's also one of the oldest language. With MySQL it provide all features required to build to complete web application.
But there are several new languages like NodeJS which are optimized for web applications. But it still require more configuration than a LAMP/LEMP stack.
I am not a developer, but I guess following are few points,
1. It's released in 1994 - at that time not much languages to develop web applications were available or those weren't easy.
2. It's more targeted tor web development and/or server side scripting, which took attention from targeted audience.
3. It's super easy to learn PHP and the basic syntax is easy to learn/write/read
4. More detailed documentation and user base
5. The development community still keeping their pace and release new versions with exciting features
6. It is easy to deploy the server infrastructure for PHP based web applications
7. May be many more points...
That said, people are tend to move to the new gen languages like Python, NodeJS, Ruby, etc to build new web applications which demands more advanced processing. But as far as I can see, people with advanced knowledge are trying them yet!
Php is popular because of its ease of use by developers. One can start from zero and be able to create meaningful products with php. Though it teaches developers some weird things if one is not proficient in programming. And secondly, a lot of businesses use it nowadays.
PHP is popular because all of its codes is applicable to most of programming languages. One thing makes it more popular is that its user friendly features. The Codes of PHP is much simplier than the other langauges. You should be aware that facebook, Wikipedia and any wordpress powered blogs are build up with PHP languages.
I think why PHP is popular is, it is one of the most oldest programming language. PHP is very flexible language, you can use it to develop many programs like web application, websites and others. It is also easy to learn and use. I remember when I was in 3rd year college, I have web programming subject and my professor start to teach us PHP. I start to learn this language easily and I loved it. Then that's the start of my journey on developing websites using PHP.
PHP boasts Simplicity
With PHP, coding is like writing an essay in English for a computer to read, except the essay is a piece of code that instructs it to create and run websites in the most efficient manner possible. Creating a code in PHP is simple, and though you'll hear some developers complaining that this does not build the necessary coding habits that streamlines (read standardizes) coding, it allows the code to be highly personalized (and hence easier to protect).
For the web development PHP is the best which runs on the Linux, UNIX and Windows platform. It has the simple syntax which programmer can easily learn. Large databases can be handled by the PHP. It can run the complex web applications too.