The “<?=" sequence (which is short for "<?php echo" is no longer supported in many recent versions of PHP (I'd specify which one specifically, but I forget… I think somewhere in the 4.x series) and should be avoided for compatibility. It's only a few extra characters to replace it with "<?php echo" (I know it's not as nice looking for simplistic code) and ensures compatibility with newer PHP installations.

Also, your last bit of code (for editable regions) defines a variable called "$left_nav" – I first thought there was a space in the variable name, but it just seems your choice of colour scheme and font size or something has caused your undscore to appear invisible in your code snippet. May want to double check that