================================================================================
check.php
================================================================================
<?php
    $host = "localhost";
    $user = "root";
    $pass = "";
    $dbname = "database_herb";
    session_start();
     $con = mysqli_connect($host,$user,$pass,$dbname) or die (mysqli_error()) ;

       $sql_login = "SELECT * FROM system_admin WHERE admin_username ='"mysqli_real_escape_string($con,$_POST["username"])."'
       AND admin_password = '"mysqli_real_escape_string($con,$_POST["password"])."'";

       $query_login = mysqli_query($con,$sql_login);
       $result_login = mysqli_fetch_array($query_login,MYSQLI_ASSOC);
       if (!$result_login) {
         echo "รหัสผ่านหรือไอดีผู้ใช้ไม่ถูกต้อง";
       }else {
         echo $result_login['admin_name']."<br/>";
       }
     mysqli_close($con);
  ?>
================================================================================
form_login.php
================================================================================
<?
include 'check.php';
?>
<form name="login" method="POST" action="check.php" >
   <label for="Username">Username</label>
    <input type="text" name="username" id="username" />

    <label for="password">Password</label>
    <input type="password" name="password" id="password" />

    <button type="submit">login</button>
</form>
================================================================================