卸载前正在更新MySQL字段

我正在开发一个CMS，允许用户访问MySQL数据库中的记录并对其进行分类。我已经为这个CMS创建了一个用户登录系统。我正在尝试编程一个锁定功能，该功能将在查看记录时进行监控，以便其他用户无法访问此记录。这是为了防止多个用户试图同时写入同一条记录。

到目前为止，这部分工作得还不错。当他们访问记录时，我会更新MySQL数据库中该特定记录的字段"in_use"，以显示它正在被查看。当他们提交了带有记录分类的表格时，我会更新字段"in_use"以显示它不再被查看。

我遇到的问题是，当用户在没有提交表单的情况下关闭浏览器时，或者当他们离开页面时。

我一直在尝试使用AJAX来运行beforeunload事件的更新，但它一直不起作用。

*****readRecord.php*****

<h1>Analyze</h1>
//PHP include that contains functions for php queries to the MySQL database.
<?php
    include 'core/database/record_functions.php';
?>
//Form to search MySQL database by Case Number and Button to manually release a record that is in_use.
<form action="" method="post">
<p id="case_search">Search records by Case Number: <input type="text" id="case_num_field" name="case_num">
<input type="text" name="in_use" id="in_use" readonly="readonly" hidden="hidden" value="1">
<input type="text" name="user" id="user" readonly="readonly" hidden="hidden" value="<?php echo $_SESSION['username']; ?>">
<input type="submit" name="Submit" value="Select" id="case_num_submit">
<input type="Submit" name="release record" value="Release the current record" onclick="<?php update_in_use($_SESSION['case_num'], 0); ?>">
</p>
</form>
//Includes for jQuery library and AJAX code for automatically releasing the record on page navigation or close.
<script type="text/javascript" src="core/jquery-2.1.3.js"></script>
<script type="text/javascript" src="ajaxBridge.js"></script>
... code continues for reading records
... when the form is submitted update_in_use is called again to set the value to 0

*****ajaxBridge.js*****

// This code is supposed to run the query on releaseVariable.php to update the in_use field to 0 on the beforeunload event.
window.addEventListener("beforeunload", function(event){
   if(xmlHttp.readyState==4)
   {
    xmlHttp.open("POST", "releaseVariable.php", true);
    xmlHttp.send();
    event.returnValue = "Hey it worked!";
   }
   event.returnValue = "Hey it failed!";
   });

*****releaseVariable.php*****

// This simply runs the update_in_use function to set the in_use field to 0 for this particular case.
<?php
include 'core/init.php';
include 'read_record.php';
protect_page();
update_in_use($_SESSION['case_num'], 0);
?>

我这样做对吗？如果不是，我该如何解决这个问题？

如果这是一个好方法，那么我在哪里误入歧途了代码？