PHP is a programming language.

Drupal is a framework built on PHP, and is marketed as a content management system.

According to Wikipedia, Drupal acts as the back-end framework for at least 2.3% of all websites internationally.

In other words, Drupal uses PHP to deliver much of its functionality. From version 8 onwards, Drupal relies on Symfony, which is a PHP-based framework. However, all versions of Drupal rely on PHP in some way.