set urls with parameter to noindex?

Hi,

i use Woocomerce for my enfold-site. for the print version of some sites i use a parameter like this http://www.mywebsite.de/kategorie/?wpp_export=print

How can i set these Urls to noindex. I tryed a code like this in the header.php:

<?php
    $url = $_SERVER['REQUEST_URI'];
    if (strpos($url,'?') !== false) {
        echo '<meta name="robots" content="noindex, follow" />' . "\n";
    }
?> 

But the code did not work. There is no “noindex” in the head of these sites…

Could you help me with this?

Best regards

Chris