注意：未定义索引如何更正

Question

我有在db中搜索的表格。当我打开搜索页面时，我收到以下消息： 注意：第90行的D：\ xampp \ htdocs \ zdrave \ search.php中的未定义索引：element_1

注意：未定义的索引：第90行的D：\ xampp \ htdocs \ zdrave \ search.php中的element_2

注意：未定义的索引：第90行的D：\ xampp \ htdocs \ zdrave \ search.php中的element_3 我该如何纠正？

    <?php
$con=mysqli_connect("localhost","root","","expences");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$result = mysqli_query($con,"SELECT inv_num, inv_date, inv_desc, inv_amnt FROM invoices WHERE inv_num = '$_POST[element_1]' or inv_date = '$_POST[element_2]' or inv_desc like '$_POST[element_3]' ");

echo "<table border='1'>
<tr>
<th>Фактура</th>
<th>Дата</th>
<th>Описание</th>
<th>Стойност</th>
</tr>";

while($row = mysqli_fetch_array($result))
{
echo "<tr>";
echo "<td>" . $row['inv_num'] . "</td>";
echo "<td>" . $row['inv_date'] . "</td>";
echo "<td>" . $row['inv_desc'] . "</td>";
echo "<td>" . $row['inv_amnt'] . "</td>";
echo "</tr>";
}
echo "</table>";

    mysqli_close($con);

?>