This is the opposite pair to ‘the front end’ ( html, css, javascript).
Known as ‘the back end’.
PHP and MYSQL exist on the ubuntu server and perform all the tasks that are located at the website.
Remember the front end occurs inside of the end users web browser, whether that be a computer, laptop, mobile phone or even video game console or smart tv.
Therefor the back end occurs at the website. In the case of the basic materials project, via WordPress, the PHP and MYSQL do there thing and combine the results into HTML CSS and JAVASCRIPT which is then sent into the end users web browser.
Connections between the front end and the back end are sometimes called AJAX. The AJAX part combines both the front end and the back end.
So, a developer who does all three parts, both the ‘front end’, the ‘back end’, and the AJAX part which connects the two , is called a full stack developer. That’s a lot of work!
It can get very complicated ! this is traditionally known as a LAMP stack.