How to connecting to MySQL database from PHP designer 8

i am new to PHP. i am trying to create a page that will input data into
mysql database. i created a database called 'form1' and a table called 'demo'
with 2 columns ID(auto Increase) and input1(varchar).
i created a html file called demo-form with this codes

!DOCTYPE HTML>

Input 1:

and a PHP file called 'demo' with code

php

/**
* @author gencyolcu
* @copyright 2015
*/

define('DB_NAME', 'form1');
define('DB_USER','root');
define('DB_PASSWORD','');
define('DB_HOST', 'localhost');

$link = mysql_connect(DB_HOST,DB_USER,DB_PASSWORD);

if (!$link) {

die('Could not connect: ' . mysql_error());
}

$db_selected = mysql_select_db(DB_NAME, $link);

if (!$db_selected) {

die('Can\'t use' . DB_NAME . ':' . mysql_error());
}

echo 'Connected successfully';

$value = $_POST['input1'];

$sql = "INSERT INTO demo (input1) VALUES ('$value')";

if (!mysql_query($sql)) {
die('Error: '. mysql_error());
}

mysql_close();

but after running the demo-form in my browser and i enter text into the
textbox and click submit.
no action takes please. dont really no what the problem is. please help me out