PAYH - Connect to database from php

[#1]

I have a plusnet (free-online) account and also a PAYH site.
I can connect to my PN database from my PN (ccgi) site and from my PAYH site.
I am trying to connect to my PAYH database from my PAYH site, but I keep getting an error.
Is there a different syntax required for connecting to the PAYH MySQL database?
I can't find any information at all on the PAYH site about this.
Here is what I am currently doing:
// free-online settings
$db_host  = "rumpus.force9.net";
// Pay as you host settings:
// $db_host  = "plesk-db01.plus.net";
$username = "XXXX";
$password = "XXXX";
$DB_name  = "XXXX";
    $chan = mysql_connect ($db_host, $username, $password);
mysql_select_db ($DB_name, $chan);

$sql = 'select ...
$result = mysql_query ($sql, $chan);
if ($myrow = mysql_fetch_array($result))
I can access the database fine using phpmyadmin through Plesk. I am using the same database name and password, and I can execute the same sql query with no problem.
The error I get from php is:
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource
Any suggestions appreciated.
TIA
Phil.

[#2]

(Having written of my deep disappointment am withholding the text of my experiences for 24 hours, to allow time for reflection, before publishing the sorry saga)

[#3]

Check the db connection in your php with:
$chan = mysql_connect ($db_host, $username, $password);
if (!$chan)
  echo "Unable to connect";
$sql = mysql_select_db(whatever, $chan);
if (!$sql)
  echo "Unable to select";
Might help narrow it down.