替换为:

$result = mysqli_query($con, "DELETE FROM Tools WHERE PartNumber = {'$_POST['pnumber']'}") or die ("Couldn't execute query: " .mysqli_error($con));

有了这个:

$result = mysqli_query($con, "DELETE FROM Tools WHERE PartNumber = '" . $_POST['pnumber'] ."'") or die ("Couldn't execute query: " .mysqli_error($con));

注意我没有处理上面的sql注入

更好的是使用可以保护您的查询的准备语句，在您的情况下，它将是这样的:

Even better is to use prepare statements that will secure your querys, in your case it will be something like this:

$sql= 'DELETE FROM Tools WHERE PartNumber= ?';      

$stmt = $con->prepare($sql); 
$stmt->bind_param('i', $_POST['pnumber']);
$stmt->execute();