The key to debugging with PHP is isolating the line of code that causes the problem. Turning on debugging can often help to give you a line number for en error, but when that doesn't work I will usually add a die() statement to the page and then move it down the page a few lines at a time to isolate the line of code that is breaking the display.
So start by adding this to the first line:
and then maybe repeat that line with a new number down the page at various points.
Then browse the page. It will show you only the first line it gets to. When you see that line, delete it and browse the page again.
By doing this you can find the general area of the error and then put die() statements closer together until you have isolated the exact line of code causing the problem. Once you have done that, it usually becomes easy to fix. (perhaps an include file you forgot to upload)