PhP Login/Register system
<?php

function register_form(){

$date = date('D, M, Y');
echo "<form action='?act=register' method='post'>"
."Username: <input type='text' name='username' size='30'><br>"
."Password: <input type='password' name='password' size='30'><br>"
."Confirm your password: <input type='password' name='password_conf' size='30'><br>"
."Email: <input type='text' name='email' size='30'><br>"
."<input type='hidden' name='date' value='$date'>"
."<input type='submit' value='Register'>"
."</form>";

}

function register(){

$connect = mysql_connect("host", "username", "password");
if(!$connect){
die(mysql_error());
}

$select_db = mysql_select_db("database", $connect);
if(!$select_db){
die(mysql_error());
}

$username = $_REQUEST['username'];
$password = $_REQUEST['password'];
$pass_conf = $_REQUEST['password_conf'];
$email = $_REQUEST['email'];
$date = $_REQUEST['date'];


if(empty($username)){
die("Please enter your username!<br>");
}

if(empty($password)){
die("Please enter your password!<br>");
}

if(empty($pass_conf)){
die("Please confirm your password!<br>");
}

if(empty($email)){
die("Please enter your email!");
}


$user_check = mysql_query("SELECT username FROM users WHERE username='$username'");
$do_user_check = mysql_num_rows($user_check);


$email_check = mysql_query("SELECT email FROM users WHERE email='$email'");
$do_email_check = mysql_num_rows($email_check);


if($do_user_check > 0){
die("Username is already in use!<br>");
}

if($do_email_check > 0){
die("Email is already in use!");
}


if($password != $pass_conf){
die("Passwords don't match!");
}


$insert = mysql_query("INSERT INTO users (username, password, email) VALUES      ('$username', '$password', '$email')");
if(!$insert){
die("There's little problem: ".mysql_error());
}

echo $username.", you are now registered. Thank you!<br><a href=login.php>Login</a> |     <a href=index.php>Index</a>";

}

switch($act){

default;
register_form();
break;

case "register";
register();
break;

}

?>

require('connect.php');

<?php
mysql_connect("localhost","host","password");
mysql_select_db("database");
?>

<form action='register' method='post'>
Username: <input type='text' name='username' size='30'><br>
Password: <input type='password' name='password' size='30'><br>
Confirm your password: <input type='password' name='password_conf' size='30'><br>
Email: <input type='text' name='email' size='30'><br>
<input type='hidden' name='date' value='$date'>
<input type='submit' name="register" value='Register'>
</form>

<?php
    if (!isset($_POST))
        register_form();
    else
        register();