can't update mysql table with $_post data - php

Question

Here is the page show table from database and every row has textbox, and b_id to update table.
$result2 = mysqli_query($con,"SELECT * FROM zahialga WHERE bid IN ($imp)");
while($row = mysqli_fetch_array($result2))
{
echo "<tr>";
echo "<td>" . $row['b_id'] . "</td>";
echo "<td>" . $row['materials'] . "</td>";
echo "<td>" . $row['num'] . "</td>";
echo "<td> <input type='text' name='sh_num[]' maxlength='10'></td>";
echo "</tr>";
echo "<input type='hidden' name='b_id[]' value='" . $row['bid'] . "'>";
}
echo "</table>";
echo "<input type='submit' value='Илгээх'/>";
this is update page.
$con=mysqli_connect("localhost","root","","login");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$sh_id_query = mysqli_query($con,"SELECT sh_id FROM zahialga order by bid desc limit 1");
$sh_id1 = $sh_id_query->fetch_object()->sh_id;
$sh_id2 = $sh_id1 + 1;
$count = count(array_filter($_POST["sh_num"]));
for($i=0;$i<$count;$i++)
{
$insert = mysqli_query($con,"UPDATE table SET
sh_id='{$sh_id2}',
sh_num='{$_POST['sh_num'][$i]}'
WHERE bid = '{$_POST['$b_id'][$i]}'");
}
this is getiing Notice: Undefined index: $b_id error. What im missed?
BTW sorry about my bad english.

Answer 1

There should be no $ sign:
$_POST['b_id'][$i]

Answer 2

$insert = mysqli_query($con,"UPDATE table SET
sh_id='{$sh_id2}',
sh_num='{$_POST['sh_num'][$i]}'
WHERE bid = '{$_POST['$b_id'][$i]}'");
// ^ additional $ sign here
should be
$insert = mysqli_query($con,"UPDATE table SET
sh_id='{$sh_id2}',
sh_num='{$_POST['sh_num'][$i]}'
WHERE bid = '{$_POST['b_id'][$i]}'");