I want to give all php files that return json the .json extension.

When I change the extension to json, I lose syntax highlighting for php.

Is there a way to solve this?

Thanks!

Try to click file type name (for JSON file will be JSON) next to current file character encoding name in status bar and choose "PHP".

Alternatively you can give "name.php.json" instead of "file.json" to the file of interest and register file mask "*.php.json" (without quotes) as an alernative file mask for the PHP file.
You can do so in Settings dialog at the "Associations" page.

The first method worked just fine. Thank you.