<?php
include('header.php');
if (!empty($submit))
{
$conn = mysql_connect("localhost","$username","$password")
or die('The user name or password you enterd are not true');
mysql_select_db("$dbname")
or die('The Database name you enterd is not true');
echo('Succesfully connected to Database');
}
else
{
echo('Please submit your details to connect the Database');
}
?>

<form name="dbconn" method="POST" action="connectdb.php">
DataBase name: <input type="text" name="dbname"><br>
User name to the DataBase: <input type="text" name="username"><br>
Password to the DataBase: <input type="text" name="password"><br>
<input type="submit" name="submit" value="submit"><br>
</form>

$submit=...

$_POST("USER"};