传递给函数的onClick的JavaScript变量未定义

我正在尝试使用PHP，JavaScript和MySQL编写一个简单的表决脚本。当将变量从href onclick参数中传递给javascript函数时，出现“未定义”错误，我相信是指该变量。

我的PHP看起来像这样：

echo "<td class=\"uparrow\"><a href=\"javascript:void(0);\" onclick=\"upVote.call(". $row['serial'] ."); return false;\">&#8689;</a></td>";

生成这样的HTML：

<td class="uparrow"><a href="javascript:void(0);" onclick="upVote.call(16); return false;">&#8689;</a>

函数看起来像这样：

function upVote(str) {
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("report_status").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","up_vote.php?serial="+str,true);
xmlhttp.send();
}

当upVote（str）函数调用up_vote.php时，由于某种原因该变量未传递给PHP。

upVote PHP如下所示，当直接通过URL访问时，功能很好：

<?php
// Create Connection
$dbconnect=mysqli_connect("127.0.0.1","user","pass","table");

// Check connection
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
} else {
//  echo "Connection Sucessful<BR><BR>";
}

// escape variables for security
if (isset($_GET['serial']))
{
    $serial = mysqli_real_escape_string($dbconnect, $_GET['serial']);
}

// define SQL call and fetch the result
$sql = "SELECT vote FROM urls WHERE serial = $serial";
$result = mysqli_query($dbconnect,$sql);

if (!$result) {
  die('Select Error: ' . mysqli_error($dbconnect));
}
//echo $sql . "<BR>"; // for error checking the SQL call

    while($row = mysqli_fetch_array($result)) {
        $return = $row['vote']+1;
    }

echo $return;  //Send result back to javascript

//Update the vote count in the database
$sql = "UPDATE urls SET vote=$return WHERE serial = $serial";
$result = mysqli_query($dbconnect,$sql);

if (!$result) {
  die('Update Error: ' . mysqli_error($dbconnect));
}

mysqli_close($dbconnect);
?>