ไม่รู้ code ผิดตรงไหน(php) ไม่ส่งค่าจาก formไฟล์ html ไปไฟล์ php

<?
//ob_start();
//session_start();	
  
    $link = mysql_connect("localhost", "root", "1234");		
    mysql_query("SET NAMES utf8", $link);
    mysql_query("Use formdb;");		

    //$name=$_POST[name];
    $username=$_POST[username];
    $password=$_POST[password];

    $usernamedb = "Select username from form where username=$username;";
    $passworddb = "Select password from form where password=$password;";
	
    if(($usernamedb)&&($passworddb)){
	    
        //session_start();
		
	session_register("usernamedb");
	session_register("passworddb");
		
	echo "<a href=admin_session.php>เข้าสู่ระบบ</a>";
    } else {
	    echo "Error Login";
    }
	
    //ob_end_flush();
?> 
	

<?
//ob_start();
//session_start();	

$link = mysql_connect("localhost", "root", "1234");	
mysql_query("SET NAMES utf8", $link);
mysql_query("Use formdb;");	

//$name=$_POST[name];
$username=$_POST['username'];
$password=$_POST['password'];

$num_row=mysql_query("SELECT * FROM tb_example WHERE username='".$username."' AND password='".$password."'");

if($num_row == 1){

//session_start();
$select=mysql_query("SELECT * FROM tb_example WHERE username='".$username."' AND password='".$password."'");
$result=mysql_fetch_array($select);
$_SESSION['username']=$result['username'];
$_SESSION['password']=$result['password'];

echo "<a href=admin_session.php>เข้าสู่ระบบ</a>";
} else {
echo "Error Login";
}

//ob_end_flush();
?>