Just make the database variables data come from the form, like:
<form method="post">
Database Server: <input type="text" name="db_server" /><br />
Username: <input type="text" name="db_user" /><br />
Password: <input type="text" name="db_pass" /><br />
Database name: <input type="text" name="db_name" /></form>
And then when parsing the form call upon the variables like this:
if(isset($_POST)) {
// Read the form data
$db_server = $_POST['db_server'];
$db_user = $_POST['db_user'];
$db_pass = $_POST['db_pass'];
$db_name = $_POST['db_name'];
// Let's try to connect to the entered database
$link = mysql_connect($db_server, $db_user, $db_pass);
if(!$link) {
echo 'Unable to login to database server.';
} else {
mysql_select_db($db_name);
# Queries go here
mysql_close($link);
}
}