PHP跟踪像素数据库插入

要跟踪哪个人打开了我的(mailchimp)电子邮件，我想添加一个跟踪像素。

像素现在生成了，但是每当我向url添加GET参数时，它就不起作用了

我的"pixel.php"代码

<?php
/* 
GET Methods 
*/
if (!empty($_GET)){
    $ip = $_SERVER['REMOTE_ADDR'];
// (Do|log) act on name
if (isset($_GET['name'])) { 
    $name = $_GET['name'];
} else{
    $name = "";
}
// (Do|log) act on mail/campagne id
if (isset($_GET['mailid'])) {
$id = $_GET['mailid'];
}else{
    $id = "";
}
// (Do|log) act on date
if (isset($_GET['date'])) {
$date = $_GET['date'];
} else{
    $date = "";
} 
insert($ip, $name, $mailid, $date);
}else{
// normal browsing to pixel.php without parameters set
// no insert here
}
/*
INSERT
*/
function insert($ip, $name, $mailid, $date){    
// Create connection
$conn = mysqli_connect("192.168.****.****", "****", "****", "maildata");
if (!$conn) {
die("Connection failed: " . mysqli_connect_error());
}
$query  =  "INSERT INTO opened (ip, name, mailid, date)
VALUES ('".$ip."', '".$name."', '".$mailid."', '".$date."')";
if (mysqli_query($conn, $sql)) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . mysqli_error($conn);
}
mysqli_close($conn);
}

/*
GENERATE IMAGE
*/
// Create an image, 1x1 pixel in size
$im=imagecreate(1,1);
// Set the background colour
$white=imagecolorallocate($im,255,255,255);
// Allocate the background colour
imagesetpixel($im,1,1,$white);
// Set the image type
header("content-type:image/jpg");
// Create a JPEG file from the image
imagejpeg($im);
// Free memory associated with the image
imagedestroy($im);
/*
call with <img src="pixel.php?userid=98798&campaign=302&last=8"> for example
*/
?>

我认为错误是在INSERT mysql语句的某个地方，但我不知道在哪里，我得到的唯一反馈是"找不到图像"-图标