CREATE TABLE users(
id INT NOT NULL AUTO_INCREMENT,
PRIMARY KEY(id),
username VARCHAR(30) NOT NULL,
password VARCHAR(20) NOT NULL);
 
-- USERS --
INSERT INTO `users` VALUES (1, 'admin', '827ccb0eea8a706c4c34a16891f84e7b');

<?php
 
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = '12345';
$db 	= 'web';
 
$conectar = mysql_connect($dbhost,$dbuser,$dbpass); 
mysql_select_db($db,$conectar);
 
if (isset($_SESSION['admin_username'])){
	echo "Ya estás autentificado";
        header("Location: admin.php");
}
 
if ($_POST['username']) {
	$username = $_POST['username'];
	$password = $_POST['password'];
 
 
	if ($password==NULL) {
		echo "La password no fue enviada";
	}
 
	else{
		$query = mysql_query("SELECT username,password FROM users WHERE username = '$username'") or die(mysql_error());
		$data = mysql_fetch_array($query);
		if($data['password'] != md5($password)) {
			echo "Login incorrecto";
		}
 
		$query = mysql_query("SELECT username,password FROM users WHERE username = '$username'") or die(mysql_error());
		$row = mysql_fetch_array($query);
		$_SESSION["admin_username"] = $row['username'];
		echo "Has sido logueado correctamente ".$_SESSION['admin_username']." y puedes acceder al admin.php.";
		}
	}
?>
 
 
<form action='login.php' method='POST'>
<table style='border:1px solid #000000;'>
<tr>
<td align='right'>
Nombre de usuario: <input type='text' size='15' maxlength='25' name='username'>
</td>
</tr>
<tr>
<td align='right'>
Password: <input type='password' size='15' maxlength='25' name='password'>
</td>
</tr>
<tr>
<td align='center'>
<input type="submit" value="Login">
</td>
</tr>
<tr>
<td align='center'>
</td>
</tr>
</table>
</form>