php tags in js file generating "Unexpected token error in console log

Question

I'm upgrading my Zencart based website to version 1.5.7 and a module that previously worked perfectly is now broken.

On inspecting the console log I'm seeing " Uncaught SyntaxError: Unexpected token '<' " The code line where it reports this error is in a js file

var session_language_id = <?php echo $_SESSION['languages_id']; ?>;

I've got other instances of the same error in different files and it's always where a <?php tag has been used inside a script.

Given that this should be a valid code format, can anyone shed any light on what might be causing this issue as it's got me stumped!