Contextual Filter Problem in Drupal Commerce Using Indexed Textfield
I think it's because the value that you put into the Views preview will just be a value to pass to the PHP code. So if you enter return "Breitling", your code is going to try and find a brand named 'return "Breitling"'.

If you just need to return all results from the brand Breitling for testing purposes, in your PHP code in the contextual filter, set the value to look for as "Breitling".

Let me know how you get on.