Jquery ajax call return php file content

I have a jQuery ajax function that asks for a php file (carousel.php) and does some things with the result. This all works fine on our development server, but on our real server the script seems to fail and returns the contents of the PHP file (source code). So PHP doesn't even get executed. What could be here?

        $.ajax({
        url: 'carousel.php',
        dataType: 'json',
        cache: false,
        success: function(result){
            if(result.success){
                if (result.numRows == 0) {
                    $('#insert').css('display','none');
                }
                $('#selectBox').html(result.html);
            } 
        }
    })