Error while trying to include external php file in functions.php or header.php

If I put following codes in any template file, it does work perfectly;

require("shop/dbconnect.php");
$totalitems = count ($_SESSION['cart']['products']) + count ($_SESSION['cart']['addons']) + count ($_SESSION['cart']['domains']);

if($totalitems > 0){
echo '</br><b>You have ' . $totalitems . ' item</b></br>';
}
else {
echo '</br><b>You have no item</b></br>';
}

But when I try to put this in a shortcode, I get error. Here is the code, I put in functions.php

function ex_whmcs(){
	require("shop/dbconnect.php");
	$totalitems = count ($_SESSION['cart']['products']) + count ($_SESSION['cart']['addons']) + count ($_SESSION['cart']['domains']);
	if($totalitems > 0){
  		echo '</br><b>You have ' . $totalitems . ' item</b></br>';
  	}
  	else {
  		echo '</br><b>You have no item</b></br>';
  	}
}
add_shortcode( 'whmcscart', 'ex_whmcs' );

Now when I call the shortcode [whmcscart], it stops after executing require(“shop/dbconnect.php”);
If I remove this line ( require(“shop/dbconnect.php”); ) from functions.php and add the shortcode in template file like this:

require('shop/dbconnect.php');
echo do_shortcode('[whmcscart]');

Then it works perfectly.

If I add require(‘shop/dbconnect.php’); in header.php , that also stops working!

My wordpress file is on https://www.domain.com and shop is another directory where WHMCS is installed.

Can anyone help me about this?
Thanks.