If you stay on the same web stack you might be able to migrate one piece at a time.
You also have the option of refactoring the most important or troublesome bits of your app to use Symfony2 while leaving the rest alone.
It's pretty challenging to take one piece of your big web application and migrate that to Python while leaving the rest in PHP. So you'll tend to do it all at once. That's very expensive, and very risky, even if your original ugly PHP site had an extensive test suite, which it will not.
You also have the option of refactoring the most important or troublesome bits of your app to use Symfony2 while leaving the rest alone.
It's pretty challenging to take one piece of your big web application and migrate that to Python while leaving the rest in PHP. So you'll tend to do it all at once. That's very expensive, and very risky, even if your original ugly PHP site had an extensive test suite, which it will not.