PHP
PHP is the most popular Web programming language because PHP is the easiest to learn, easiest to use, fastest to develop, and the fastest to run working code. PHP is a language that everyone can use from a really simple beginning up to the most complicated applications. There is no reason to use anything else for Web sites and the servers behind apps.
Back in the last century, people attempted to use compiled languages to generate Web site content but ran into endless difficulties. The classic several expensive Java programmers working for months instead of one PHP programmer for a few weeks. PHP changes development from a huge formal project to something you can do while sitting with the customer.
Back in the last century, people tried to use scripting languages like the unreadable Perl and the unreliable Mocha. Mocha was renamed to LiveScript to emphasise that the language was not a dead compiled language. Netscape renamed LiveScript to JavaScript to pretend JavaScript could replace Java.
JavaScript survived in the Web browser because there was nothing else. Up on the Web server, PHP replaced everything else because PHP solved more problems than what it created.
PHP 3 connected to more server functionality and databases than any other language. PHP 4 introduced code structure options to handle huge applications. PHP 7 made PHP close to machine speed.
How does PHP work so well? PHP is both an interpreted language and a compiled language. PHP achieves the best of both worlds by internally compiling the code to a very fast form. The first time you run code, it works like an interpreted language giving you instant updates without waiting for a massive compile process. Internally the result is saved so that every other use of the code works as a compiled language.
Back when Java was a slow old interpreted language, Microsoft brought out a compiled version of Java that was 20 times faster than any previous Java. Unfortunately the Microsoft version still required the compilation process. They never caught up to PHP.
In 1992, Linus Torvalds created the modern open source software movement when Linus released Linux. In 1994, Rasmus Lerdorf invented the first version of PHP, giving us the chance to build a Web server application using open source software. 1995 added MySQL and PostgreSQL for the database part of the Web server.
The third version of PHP, PHP 3, expanded PHP do do everything needed for most Web sites. This century started with the release of PHP 4, completing the coverage of everything everyone needed for a Web site and structures for much larger applications. 2004 introduced PHP 5 for better object oriented structure . 2015 ended with the release of the super fast PHP 7.