<?php  
$connect = mysql_connect('localhost','root','')or die(mysql_error());  
mysql_select_db('prepod');  

if(isset($_POST['submit'])){  
$username=$_POST['username'];  
$login=$_POST['login'];  
$password=$_POST['password'];  
$r_password=$_POST['r_password'];  
if($password == $r_password){  
$password = md5($password);  

$query = mysql_query("INSERT INTO users VALUES ('$username','$login','$password')") or die(mysql_error());  
}  
else{  
die('Пароли не совпадают!');  
}  
}  

?>  
<form method="post" action="register.php">  
<input type="text" name="username" placeholder="| Имя" required/><br>  
<input type="text" name="login" placeholder="| Логин" required/><br>  
<input type="password" name="password" placeholder="| Пароль" required/><br>  
<input type="password" name="r_password" placeholder="|Повтор пароля" required/><br>  
<input type="submit" name="submit" value="register"/>  
</form>