undefined index on $_SESSION and all $_POST part - php

Question

I want to make register form for my website. But I faced error and this is my code:
<?php
/* Registration process, inserts user info into the database
and sends account confirmation email message
*/
// Set session variables to be used on profile.php page
$_SESSION['email'] = $_POST['email'];
$_SESSION['first_name'] = $_POST['firstname'];
$_SESSION['last_name'] = $_POST['lastname'];
$mysqli = mysqli_connect("localhost","root","","data");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
// Escape all $_POST variables to protect against SQL injections
$first_name = $mysqli->escape_string($_POST['firstname']);
$last_name = $mysqli->escape_string($_POST['lastname']);
$email = $mysqli->escape_string($_POST['email']);
$password = $mysqli->escape_string(password_hash($_POST['password'],
PASSWORD_BCRYPT));
$hash = $mysqli->escape_string( md5( rand(0,1000) ) );
// Check if user with that email already exists
$result = $mysqli->query("SELECT * FROM login WHERE email='$email'") or
die($mysqli->error());
// We know user email exists if the rows returned are more than 0
if ( $result->num_rows > 0 ) {
$_SESSION['message'] = 'User with this email already exists!';
header("location: error.php");
}
else { // Email doesn't already exist in a database, proceed...
// active is 0 by DEFAULT (no need to include it here)
$sql = "INSERT INTO login (first_name, last_name, email, password, hash)"
."VALUES ('$first_name','$last_name','$email','$password', '
$hash')";
// Add user to the database
if ( $mysqli->query($sql) ){
$_SESSION['active'] = 0; //0 until user activates their account with
verify.php
$_SESSION['logged_in'] = true; // So we know the user has logged in
$_SESSION['message'] =
"Confirmation link has been sent to $email, please verify
your account by clicking on the link in the message!";
// Send registration confirmation link (verify.php)
$to = $email;
$subject = 'Account Verification ( clevertechie.com )';
$message_body = '
Hello '.$first_name.',
Thank you for signing up!
Please click this link to activate your account:
http://localhost/login-system/verify.php?email='.$email.'&
hash='.$hash;
mail( $to, $subject, $message_body );
header("location: profile.php");
}
else {
$_SESSION['message'] = 'Registration failed!';
header("location: error.php");
exit();
}
}
The error state that:
Undefined index: firstname, lastname, email, password
at line set session variables and line for escape all $_POST variables to protect SQL injection. Anyone know how to fix this error. Thank you.

Answer 1

You are missing session_start() at the start of your code, this needs to be called before you echo/output anything to the browser.
You'll also need to add session_start to your error.php to read from the session there.
When session_start() is called or when a session auto starts, PHP will call the open and read session save handlers...
The read callback will retrieve any existing session data (stored in a special serialized format) and will be unserialized and used to automatically populate the $_SESSION superglobal when the read callback returns the saved session data back to PHP session handling.
See: http://php.net/manual/en/function.session-start.php

Answer 2

If(isset($_POST['first_name']) && isset($_POST['last_name']) && isset($_POST['email'])){
// Escape all $_POST variables to protect against SQL injections
$first_name = $mysqli->escape_string($_POST['firstname']);
$last_name = $mysqli- >escape_string($_POST['lastname']);
}
In PHP, a variable or array element which has never been set is different from one whose value is null; attempting to access such an unset value is a runtime error.
That's what you're running into: the array $_POST does not have any element at the index "username", so the interpreter aborts your program before it ever gets to the nullity test.