I've just upgraded to build 5210 and can no longer type. Critical keys like backspace, delete, enter and the cursor keys no longer work. I can still type letters and numbers etc, but can't really edit anything, as I can't move the cursor back or delete anything I've typed.

Yep, it's happened to a few of us however, try a reboot.

Rebooting did not help.

However, I have resolved the problem.
For some reason, after the upgrade PhpEd had added an extra set of Editor Shortcuts called "Custom". This did not define keys for anything really. I changed the key mapping set back to "Default" and everything worked again.

The only think I can think of is that I had changed a couple of key mapping in the past and created the Custom set and the installer somehow did not move over the custom key mappings to the new version, but did move over the setting that I was using a custom set.

Indeed 5209 had problems with this and made the Custom set of shortcuts empty. So it remained empty when you installed 5210.

We fixed this problem in 5210, it won't make Custom set empty.