Custom text field validation

Is it possible to do custom field validation in PHP? I didn't see it as an option under Validation Rule.

Yes, the CB Code Field provides Code Validation where you can supply custom PHP for field validation. As a side note CB Query Field provides Query Validation where you can provide custom SQL for validation.

Thanks. Any advice on debugging the PHP code? 
I use PHPStorm to debug my own components. Can I connect it to this somehow?

Or is there an example of this you can point me to?

You can use a custom php file then use an include to include it into the code field. That might allow debug tools to work with it, but I'm not sure since it's string evaluation of code. Otherwise no debug tools likely won't work with it.

Thanks. As you suspected, I couldn't connect the debugger. But debugged by steps. Evidently, you catch errors and turn them into invalid input. It would be great to see the errors, some sort of debug mode.

My question now is that I can't seem to change the input value in the validation routine. If I change $value and return TRUE, it doesn't change it. Is there a way to do it?

To be clearer, I'm validating a phone number input and I want to transform it into a standard format. I can validate it but not change it.

You can't change the fields value during validation as it's just for validating the supplied value and nothing more. To change a fields value you'd have to process it as part of the storage process either using a custom CB plugin or CB Auto Actions. There's a few triggers this can be done one. They're as follows.

onBeforeprepareFieldDataSave
That is fired before a field is saved. So it can be used to modify the fields value directly in the $postdata before it ever touches the field. It's a good way to modify the field before server side validation even happens.

onAfterprepareFieldDataSave
This fires after it has already been bound to the user object so at this point you'd need to modify the value on $user. Either will work and will depend on where in the store process you want to insert custom logic.

If you want client side formatting we do support Input Masks. These let you enforce a format on a field. They're only available for Text fieldtype. You should find the parameters for this under Parameters > Display.