PHP is an open-source programming language designed for server-side web development: it generates pages and processes data before sending them to the browser.
Created in 1994 by Rasmus Lerdorf, it took off with the growth of the web thanks to its simplicity and availability at every hosting provider. It runs a considerable share of the web, including WordPress, Drupal and MediaWiki (the engine behind Wikipedia). Facebook was built in PHP in its early days.
Today's PHP has little in common with that of the 2000s: typing, mature object-oriented programming, much better performance. It is mostly used with frameworks such as Symfony and Laravel, and a dependency manager, Composer.
Its reputation is still shaped by old, unstructured code, whereas the modern language makes it possible to build robust applications.
Why it matters when hiring
The PHP talent pool is large but very uneven: a WordPress developer and a Symfony developer designing tested APIs do different jobs. Specify the context in the job ad and check in the interview their practice of testing, typing and recent versions of the language. A good modern PHP developer is often easier to hire than a Go or Rust profile of equivalent level.