SQL query according to submit form - php

Question

I want to query the database according to the input of the amount deposited. In the database the bank minimal amounts are stored. So the query consists in showing all banks with min_amount
The amount is defined $amount, and the row containing the minimum amounts is minimum. So i have done it like this but it tells me: Undefined index for $amount. ???
<form name="test" method="get" action="this.php">
Amount: <input name="amount"/>
<input type="submit"/>
</form>
<?php
$amount = $_GET["amount"];
$result = mysql_query("SELECT * FROM list1 WHERE minimum <= '$amount'");
while($row = mysql_fetch_array($result))
{
echo $row['bank_name'] . " - " . $row['Tariff'];
echo "<br />";
}
?>

Answer 1

You need to check if the form has been submitted or not. Try:
if( isset($_GET['amount'])) { /* your code here */ }

Answer 2

You need to check if the form has been submitted with say isset function.
if (isset($_GET['amount'])){}
Also it is highly advisable that you escape your $_GET variables before placing them into a mysql query as this kind of code allows for SQL injections.
The whole thing should like this:
if (isset($_GET['amount'])){
$amount = $_GET["amount"];
$result = mysql_query("SELECT * FROM list1 WHERE minimum <= '{$amount}'");
while($row = mysql_fetch_array($result))
{
echo $row['bank_name'] . " - " . $row['Tariff'];
echo "<br />";
}