Wow. Okay, so to diagnose it I copied the code in the file and then deleted manage.php altogether.
I then created a new file using the Windows Explorer GUI (the 'normal' way) called manage.php and pasted the code.
Then it worked.
Before I had created manage.php with Windows PowerShell using
new-item -type file manage.php
I have had similar problems where the encoding of PowerShell is not UTF-8 but Big-Endian or something and it...simply put, messes with things. I suppose this is a rare problem but a good lesson in diagnosing problems.
Thanks guys.