if(preg_match("/S+/", $_POST['username']) === 0){
    echo $errors['username'] = "* First Name is required.";
}

if(preg_match("/.+@.+..+/", $_POST['email']) === 0){
    echo $errors['email'] = "* Not a valid e-mail address.";
}

if(strcmp($_POST['password'], $_POST['confirm_password'])){
    echo $errors['confirm_password'] = "* Password do not much.";
}

$usercheckquery = "SELECT * FROM users WHERE username = '$_POST[username]'";
$usercheckresult = mysql_query($usercheckquery);
$data = mysql_fetch_array($usercheckresult);
    if($data[0] > 1) {
        echo "username Already in Exists</br>"; }
$emailcheckquery = "SELECT * FROM users WHERE email = '$_POST[email]'";
$emailcheckresult = mysql_query($emailcheckquery);
    $data = mysql_fetch_array($emailcheckresult);
        if($data[0] > 1) {
                echo "email Already in Exists</br>"; }

<form method="post" action="">
<p>username :
<input type="text" name="username"></p>

<p>password :
<input type="password" name="password"></p>

<p>re-type password :
<input type="password" name="confirm_password"></p>

<p>email :
<input type="text" name="email"></p>

<input type="submit" name="register" value="submit">
<a href="index.php">log in</a>

</form>

$errors = array();

if($_SERVER['REQUEST_METHOD'] == 'POST'){
//a1
if(preg_match("/S+/", $_POST['username']) === 0){
    echo $errors['username'] = "* First Name is required.</br>";
}

if(preg_match("/.+@.+..+/", $_POST['email']) === 0){
    echo $errors['email'] = "* Not a valid e-mail address.</br>";
}

if(strcmp($_POST['password'], $_POST['confirm_password'])){
    echo $errors['confirm_password'] = "* Password do not much.</br>";
}

     if (isset($_POST['register'])) {

$username = mysql_real_escape_string($_POST['username']);
$password = mysql_real_escape_string($_POST['password']);
$email = mysql_real_escape_string($_POST['email']);

//a2
$usercheckquery = "SELECT * FROM users WHERE username = '$_POST[username]'";
$usercheckresult = mysql_query($usercheckquery);
$data = mysql_fetch_array($usercheckresult);
    if($data[0] > 1) {
        echo "username Already in Exists</br>"; }
$emailcheckquery = "SELECT * FROM users WHERE email = '$_POST[email]'";
$emailcheckresult = mysql_query($emailcheckquery);
    $data = mysql_fetch_array($emailcheckresult);
        if($data[0] > 1) {
                echo "email Already in Exists</br>"; }
else {
      $query = "INSERT INTO `users` (username, password, email) VALUES     ('$username', '$password', '$email')";
      $result = mysql_query($query);
      if($result){
        echo "User Created Successfully.";
        }
    }
}
}