There are some questions for which the syntax highlighting is not active because none of the tags used for the question have syntax highlighting enabled.
- Difficulties with the #ajax form item attribute
- Show the value of a node-form field inside an AJAX field added via hook_form_alter()
- how to display the uploaded zip file content in drupal?
- How to use Devel module to see what creates an URL?
It is probably better to set PHP as default syntax highlighting mode, as in most of the questions, the shown code is PHP code; if the answer is showing code in a different programming language, it still possible to force the highlighter to use a different syntax mode, or set a different default highlighting mode for specific tags.