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
As side note, there is php-classes that can be used about PHP classes implemented in Drupal.