There is are also the problems of performance and portability. Many things are often quicker for PHP to do after requesting data from MySQL than to run using stored procedures in MySQL alone. As for portability, it can be more complicated to move your applications from one MySQL server instance to the next. There is also the possible problem that such an extension would be a effectively a direct connection to the database server. We all know the issue of SQL injection when user input isn't properly filtered and validated before being sent to the database. Now we would either have to add a new application layer inside Apache to handle this or have the database open to such issues.
If it can overcome the portability, performance, and SQL injection problems, I do see one space where it can be useful. Web services are becoming more and more common. This is a great idea to make the services more accessible. But even there, PHP is already very capable as a web service provider. It will take more than just being the "cool new thing" to draw me away from a language that I am very comfortable using.