PHP can't delete from table

I'm trying to delete data from a MySQL table. The data is inserted in a form and when the user presses submit, it should delete the data from table. This is my code,but it doesn't work. It's always showing up the error message, however, if I use id instead of key, it works just fine. Can someone help?

<?php
   include("config.php");
   session_start();

   if($_SERVER["REQUEST_METHOD"] == "POST") {

      $mykey = $_POST['proxyKey'];
      $sql = "DELETE FROM privateKeys WHERE key = '$mykey'";

    if(mysqli_query($db,$sql)) 
    {         
        header("location: PrivateList.php");
    }
    else 
    {
        $error = "Your Key is not valid";
    }
   }
?>


<html>
<head>
<title>Private Proxies</title>
<style type = "text/css">
    body 
    {
        font-family:"Lucida Console";
        font-size:25px;
        color:#f9fbff;
    }
   .box
    {
        border:#666666 solid 1px;
        width:240px;
        height:30px;
    }
</style>
</head>


<body bgcolor=#1b1b1c>

<div align = "center">
    <div style = "width:300px; border: solid 1px #333333; " align = "left">
        <div style = "background-color:#333333; color:#FFFFFF; padding:3px;"><b>Insert private Key</b></div>    
        <div style = "margin:30px">

            <form action = "" method = "post">
                <label>Key  :</label><input type = "text" name = "proxyKey" class = "box"/><br /><br />
                    <input type = "submit" value = " Submit "/><br />
            </form>
            <div style = "font-size:11px; color:#cc0000; margin-top:10px"><?php echo $error; ?></div>

         </div>

    </div>

</div>

</body>
</html>

Solution

key is a reserved keyword in MySQL and needs to be escaped by backticks.

DELETE FROM privateKeys WHERE `key` = '$mykey'
          here----------------^---^