And if that doesn’t help, add loads of echo statements between each line, just echo something different each time so you can see how far through the code you get before everything stops, then you know where to be looking. For example:
$debug = true; // shall we display debugging messages?
$host="localhost"; // Host name
$username=""; // Mysql username
$password=""; // Mysql password
$db_name="RadReq"; // Database name
$tbl_name="members"; // Table name
if ($debug) echo "Starting up";
// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
if ($debug) echo "Past server connection";
mysql_select_db("$db_name")or die("cannot select DB");
if ($debug) echo "Selected db";
// username and password sent from form
if ($debug) print_r($_POST);
... and so on.
If you’re starting out, you should also look at mysqli or PDO libraries as a way of accessing your data. PDO and prepared statements will help out with any data sanitising problems, and the older mysql functions that you use in your code are being phased out. Just doesn’t seem much point learning functions that won’t be supported soon.