PHP Sending Duplicate Emails - php

Question

After some more troubleshooting I believe I found the problem. We use QR Tags for our product and when a QR code is scanned it takes the user to the URL that runs this script. If I manually type in the URL or if I use our custom built QR scanner app then the user will receive one email. However if I user any other QR scanning app then it will send multiple emails. How can I make it so that this script will run only once each time the URL is loaded even if its from a third party app?
<?php
$queryString = $_SERVER['QUERY_STRING'];
$count=-6;
$id=substr($queryString,$count,6);
//db connection
$db = new mysqli('localhost', '*****', '*****', '*****');
if (mysqli_connect_errno()) {
echo 'Error: Could not connect to database. Please try again later.';
exit;
}
$query = "SELECT * FROM `****` where id = '$id'";
$result = $db->query($query);
$row = $result->fetch_assoc();
$email = $row['email'];
$ownername = $row['ownername'];
$petname = $row['petname'];
//check to see if tag has been registered
if ($email != "") {
//send email
$datetime = date("D M j G:i:s T Y");
$subject = "Alert";
$mailheader.= "From: " . "Tag Team <support#tag.com>\n";
$mailheader.= "X-Sender: " . "support#tag.com\n";
$mailheader.= "Return-Path: " . "support#tag.com\n";
$mailheader .= "Bcc: support#tag.com";
$body .= "Dear " . $ownername . ", \n\n";
$body .= "" . $petname . "'s Tag has just been scanned.\n\n";
$body .= "Click here to Login :\n";
$body .= "http://www.tag.com\n";
$body .= "********************\n\n";
$body .= "Regards,";
$body .= " \n\n";
$body .= "Tag Team";
$body .= " \n\n";
$body .= "Keeping Pets Safe and Found";
mail($email, $subject, $body, $mailheader ) or die ("Mail could not be sent.");
//end email alert
}
header("Location: http://www.smartphonepettag.com/id/profile.php?id=$id");
mysql_close($db);
?>

Answer 1

In the code snippet I cannot see any reason why your script should be executed more than once but relating to your post yesterday it seems as if something on your mail server is going terribly wrong.
But anyway if it's not an mail server fault the solution would be something like this:
// add this at the very first line
session_start();
// add this in the code
if($_SESSION['send'] != true){
mail($email, $subject, $body, $mailheader ) or die ("Mail could not be sent.");
$_SESSION['send'] = true;
}
This will make sure that the "mail()" function will never be executed twice for the same user.
You can learn more about Session Variables at the PHP manual.

Answer 2

You could create a flag in your database indicating if the email has been sent. Check the flag before sending the email, set it after you send the email.