Drupal executor creates a form page which allows site administrators to run custom PHP code.
It helps not only to debug code, but also to build faster your code.
It is also extremely useful for executing database queries or shell commands in production environments, in cases where the development team has no access to the console shell.
In order to activate it, just enable it from modules list, and give permission use drupal executor to any role you want to use it. Then, go to Site configuration -> Drupal executor page.
Drupal executor is not further available since Devel module includes a similar built in feature. Use Devel module and access to your site url "devel/php".