<?php

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

if(!empty($_POST['full_name']) && !empty($_POST['email']) && !empty($_POST['username']) && !empty($_POST['password'])) {
$full_name=$_POST['full_name'];
$email=$_POST'email'];
$username=$_POST['username'];
$password=$_POST['password'];
$query=mysql_query("SELECT * FROM usertbl WHEREusername='".$username."'");
$numrows=mysql_num_rows($query);
if($numrows==0)
{
$sql="INSERT INTO usertbl
(full_name, email, username,password)
VALUES('$full_name','$email', '$username', '$password')";
$result=mysql_query($sql);
if($result){
$message = "Account Successfully Created";
} else {
$message = "Failed to insert data information!";
}
} else {
$message = "That username already exists! Please try another one!";
}
} else {
$message = "All fields are required!";
}
}
?>

<?php if (!empty($message)) {echo "<p class=\"error\">" . "MESSAGE: ". $message . "</p>";} ?>
(Добавление)
11 строка точнее!!!!

таких тем было миллион, попробуйте поискать решения сам.