PHP is sripting language for creating dynamic web pages and web applications.
It is executed by the web server. Result of execution of PHP scripts is HTML content served to a client.
It can be embedded into HTML code. It is often used in combination with a MySQL database.
PHP is the most popular scripting language for web servers. Interpreter for PHP is an open source project. The interpreter is written in C language.

Millions of online stores, blogs and other web sites are based on PHP.
To name just the few:
eCommerce: Magento, PrestaShop, Shopify and WooCommerce for eCommerce.
Content management systems (CMS): WordPress, Yoomla, Drupal

Some of the most popular programming frameworks:

  • Symfony 2
  • Zend
  • Laravel
  • Phalcon
  • Yii Framework
  • Codeigniter
  • CakePHP 3.0.

Some of the most popular templating systems:

  • Smarty
  • Twig
  • Volt
  • Blade
  • Mustache