registerationphp

<?php
session_start();
include('database_1.php');
if(isset($_POST['submit']))
{
  if(preg_match("/^([a-zA-Z0-9])+([a-zA-Z0-9._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9._-]+)+$/", $_POST['email']))
  {
   $email= $_POST['email'];
   $sql1 = "SELECT * FROM users WHERE email = '$email'";
   $result1 = mysqli_query($mysqli,$sql1) or die(mysqli_error());
   if (mysqli_num_rows($result1) > 0)
            {
    echo "This email has already been used";
   }

elseif($_POST['username'])
{
$username= $_POST['username'];
   $sql2 = "SELECT * FROM users WHERE username = '$username'";
   $result2 = mysqli_query($mysqli,$sql2) or die(mysqli_error());
   if (mysqli_num_rows($result2) > 0)
            {
    echo "This username has already been used";
   }

else
 {
  $username = $_POST['username'];
  $email = $_POST['email'];
  $passwords = $_POST['password'];
  $password = md5($passwords);
 $activation = md5(uniqid(rand()));
  $sql3 = "INSERT INTO users (username, email, password, activation) VALUES ('$username', '$email', '$password', '$activation')";
  $result3 = mysqli_query($mysqli,$sql3) or die('Error: ' . mysqli_error($mysqli));
  mkdir("FilesUploading/$username");



  if($result3)
  {

   $to = $email;
   $subject = "Confirmation from CloudStockpile to $username";
   $header = "CloudStockpile: Confirmation from CloudStockpile";
   $message = "http://www.cloudstockpile.hostoi.com/Confirm.php?passkey=$activation";

   $sentmail = mail($to,$subject,$message,$header);

   if($sentmail)
            {
   echo "<b/>Hello: $username";
   echo "<br/> Your email is: $email";
   echo "<br/>Please copy and paste the link below in your browser to activate your account";
   echo "<br/> $message ";
   echo '<br /><a href="index.html">Return to Cloudstockpile login page</a>';
   }
   else
         {
    echo "Cannot send Confirmation link to your e-mail address";
   }
}
}
}
}
}
?>