How to enable the php option in Views contextual filters, provide a default value option? I have a text field called campaign and I want to set a contextual filter for this field, but I don't see a PHP option here. How can I turn this feature on? I don't see it anywhere.
You can php option in the filte using "Views Php" module  

For installing this please follow such steps :
1. Download this module using https://www.drupal.org/project/views_php.
2. Install this module.
3. In views filter now you will get PHP option.