php is used from 153 questions. Considering that Drupal is a PHP CMS, the tag should be burninated. It could be used for many different questions, such as:
- Questions about a Drupal function
- Questions about specific code required to implement a specific feature
- Questions about a PHP code used for a module that doesn't work as it is currently implemented
- Questions about the PHP code used for a theme that doesn't work as expected
php would be used in many questions asked on Drupal Answers, as Drupal is written in PHP. There are also questions about JavaScript, but there are less questions about Javascript that are specific to Drupal than questions about Drupal that could be tagged php simply for the fact the question is about PHP code used for a Drupal theme, module, or distribution.
As side note, there is php-classes that can be used about PHP classes implemented in Drupal.