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<?php
//if the form was submit
if(isset($_POST['submit'] && !empty($_POST['username']))){
  $username = $_POST['username'];
  $password = $_POST['password'];

		try{//connection to database(some database)
			$dbh = new PDO("mysql:host=$host;dbname=$dbname;charset=utf8",$username,$password,$options);
			$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
			$dbh->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);

      //check if the username exists and the password is the same as in the database
      $sql="SELECT * FROM users WHERE username=$username AND password=$password";//match username and password
      $result = $dbh->prepare($sql);
      $result->execute();
      if($result){
        return;// (what ever we want to make the connection)
        //we can set cookie/session or what ever we wish to make the username logged.
      }
      return false;//username or password not match, no connection

		}
		catch(PDOException $e){
			throw new Exception($e->getMessage());
		}
	}
 }

?>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Login</title>
</head>

<body>
  <form action="" method="POST">
    <label>Username:</label><input type="text" name="username" /><br/>
    <label>Password:</label><input type="password" name="password" /><br/>
    <input type="submit" name="submit" value="Connect" />
  </form>
</body>

</html>