There was some sort of weird corruption in your login.php file. A hidden character of some sort on line 1.
I fixed the problem by:
1) creating a new file
2) copy and pasting the entire login.php page into a text document
3) copying the code from the text file to the new document
4) saving over the login page with the new document
5) uploading the login page
Technically this should have resulted in the exact same page since I didn't change anything, but it seems to have corrected the issue.