app
  -lib
     -config.php
     -database.php
     -auth.php
     -user.php
  -index.php


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database.php

<?php

require_once('config.php');

class MySQLDatabase{

    private $connection;

    public function __construct(){

        $this->connect_database();
    }

    public function connect_database(){

        $this->connection = mysql_connect("localhost", "root", "password");
        if (!$this->connection){

            die("Cannot connect to database " . mysql_error());

        } else {

            $select_db = mysql_select_db('portal');

            if(!$select_db){

                die("Error in selecting database " . mysql_error());
            }
        }
    }

    public function query($sql){

        return mysql_query($sql, $this->connection);

    }

    public function fetch_array($result_set){

        return mysql_fetch_array($result_set);

    }

    public function num_rows($result_set){

        return mysql_num_rows($result_set);

    }

    public function close_connection(){

        if(isset($this->connection)) {
            mysql_close($this->connection);
            unset($this->connection);
        }

    }

}

$database = new MySQLDatabase();

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user.php

<?php

require_once('database.php');

class User {

    public function insert($tablename, $username, $password, $email_address){

        $result = $this->query("INSERT INTO $tablename(username, password, email_address) VALUES('$username','$password','$email_address')");
        if($result){

            return TRUE;

        } else {

            return FALSE;

        }

    }



}

----------------------------------------------------------------------------------------------

auth.php


<?php
require_once('database.php');


class Auth extends MySQLDatabase{

    public static function authenticate_user($username, $password){
        global $database;
        $sql = "SELECT username, password FROM users ";
        $sql .= "WHERE username ='" . $username ."' ";
        $sql .= "AND password ='" . $password ."'";


        $result_set = $database->query($sql);
        if(!$result_set){

            die(mysql_error());

        }
        if($database->num_rows($result_set) == 1){

            return TRUE;

        }

        return FALSE;
    }


}

$auth = new Auth();



---------------------------------------------------------------------------------------------

index.php

<?php
require_once('lib/config.php');
require_once('lib/database.php');
require_once('lib/auth.php');
require_once('lib/user.php');

if($_POST['submit']){

    $check_auth = $auth->authenticate_user($_POST['username'] ,$_POST['password']);

    if($check_auth){

        echo "welcome";


    }

}
?>
<form method="post" action="index.php" >
<fieldset>
<legend accesskey="l">Login</legend>

<label for="username">Username :</label>
<input type="text" name="username" />
<label for="password">Password :</label>
<input type="password" name="password" />
<input type="submit" value="login" name="submit" />
</fieldset>

</form>