how to insert a value in mysql using a variable code example

Example 1: how to put value in variable mysql

SET variable = (SELECT SUM(tax) FROM taxable_transactions);

Example 2: mysql add to value

<?php
$con = mysql_connect("localhost","peter","abc123");
if (!$con)
  {
  die('Could not connect: ' . mysql_error());
  }

mysql_select_db("my_db", $con);

mysql_query("UPDATE `some_table` SET `value` = `value` + 1000 WHERE `id` = 1");

mysql_close($con);
?>