PHP mysql query returns a blank screen - php

Question

I've commented out the bottom part, and the SQL query works fine. Its the displaying of the query where the error is coming from i believe.
$host = "127.0.0.1";
$user = "root";
$pass = "Toom13371!";
$connection = mysql_connect($host,$user,$pass) or die (mysql_errno().": ".mysql_error()."<BR>");
// 2. Selecting DB.
$dbname = "filters";
mysql_select_db($dbname);
// 3. Build/Test SQL Query
$sql = ("select * from filter_bandpass where start_passband=" . $_POST['Lowfreq'] . " and stop_passband='" . $_POST['Highfreq'] . "'");
//echo $sql; //Comment/Uncomment to test sql query.
// 4. Retrieve info from MySQL.
$query = mysql_query($sql);
// 5. Display Query.
echo "<table border='1'>
<tr>
<th>Low Frequency</th>
<th>High Frequency</th>
</tr>";
while($row = mysql_fetch_array($query)) {
echo "<tr>";
echo "<td>" . $row['Lowfreq'] . "</td>";
echo "<td>" . $row['Highfreq'] . "</td>";
echo "</tr>";
}
echo "</table>";
?>
Any help would be appreciated, I'm sure it's going to be some small stupid error i've over looked.
Thanks in advance :)

Answer 1

I'm guessing, based on your query, that you need to change this
mysql_select_db($dbname);
to
mysql_select_db($dbname, $connection);
and
while($row = mysql_fetch_array($query)) {
echo "<tr>";
echo "<td>" . $row['Lowfreq'] . "</td>";
echo "<td>" . $row['Highfreq'] . "</td>";
echo "</tr>";
}
to
while($row = mysql_fetch_array($query)) {
echo "<tr>";
echo "<td>" . $row['start_passband'] . "</td>";
echo "<td>" . $row['stop_passband'] . "</td>";
echo "</tr>";
}

Answer 2

Change line
mysql_select_db($dbname);
to
mysql_select_db($dbname, $connection);
Also before query check
$_POST['Lowfreq'] and $_POST['Highfreq']
If there is no value in these variables query will must return empty.

Answer 3

In your query there should not brackets for string.
$sql = ("select * from filter_bandpass where start_passband=" . $_POST['Lowfreq'] . " and stop_passband='" . $_POST['Highfreq'] . "'");
should be:
$sql = "select * from filter_bandpass where start_passband=" . $_POST['Lowfreq'] . " and stop_passband='" . $_POST['Highfreq'] . "'";