Registration Not Applying to SQL - php

Question

I Tried to test the Registration Form to See if it would work, but every time it never actually applies to the SQL Database, Is there anything that seems to be wrong in this code, i don't seem to detect any errors, i also added the registration form in html i was using below the PHP Code
<?
session_start();
include "mysqli_config.php";
$b = $_POST['username'];
$password = md5($_POST['password']);
$a = $_POST['email'];
$username = mysqli_real_escape_string($mysqli, $b);
$email = mysqli_real_escape_string($mysqli, $a);
$c = $_POST['method'];
$method = mysqli_real_escape_string($mysqli, $c);
if ($username == NULL or $email == NULL or $password == NULL) {
echo "Please Fill Out All Forms";
} else {
if (strlen($username) <= 8 || strlen($username) >= 16) {
echo " - Your username must be between 8 and 16 chars";
} else {
if ($method == NULL) {
echo "Please Select a Payment Method";
} else {
$check = "SELECT * FROM `users` WHERE `username` = '$username'";
$checksystem = $mysqli->query($check);
if (mysqli_num_rows($checksystem) != 0) {
echo "Username Already In Use!";
} else {
$create_member = "INSERT INTO `users` (`id`,`username`, `password`, `email`,`status`,`payment`)
VALUES('','$username','$password','$email','$status','$method')";
$create = $mysqli->query($create_member);
echo "Thank You For Registering, Please <a href=loginform.php>Login Here</a>";
}
}
}
}
?>
<form action="authenticate.php" id="contact" method="post" name="contact">
<div class="cleaner h10"></div><label for="author">Username</label>
<input class="required input_field" id="author" name="username" type=
"text"> <label for="email">Password</label> <input class=
"required input_field" id="email" name="password" type="password">
<label for="email">Email</label> <input class="required input_field"
id="email" name="email" type="text"> <label for="email">Payment
Email</label> <input class="required input_field" id="email" name=
"payment" type="text"> <label for="email">Use Amazon For
Payments</label> <input class="required input_field" id="email" name=
"method" type="checkbox"> <label for="email">Use Paypal For
Payments</label> <input class="required input_field" id="email" name=
"method" type="checkbox">
<div class="cleaner h10"></div><input class="submit_btn float_l" id=
"submit" name="submit" type="submit" value="Register">
</form>

Answer 1

If you trying to register a new user, first make sure the id is AUTO_INCREMENT and also
change the query
$create_member = "INSERT INTO `users` (`id`,`username`, `password`, `email`,`status`,`payment`)
VALUES('','$username','$password','$email','$status','$method')";
to
$create_member = "INSERT INTO `users` (`username`, `password`, `email`,`status`,`payment`)
VALUES('$username','$password','$email','$status','$method')";