php - Notice: Undefined index: variable

I am trying to build the following form:

  Name 
  
  

  Email 
  
  


  I need some information regarding:
  

  
  *What is 2+2? (Anti-spam)
  
  
  

The PHP code:

  $name = $_POST['name'];
  $email = $_POST['email'];
  $message = $_POST['message'];
  $from = 'From: TangledDemo';
  $to = 'hello@emailaddress.com';
  $subject = 'Hello';
  $human = $_POST['human'];

  $body = "From: $name\n E-Mail: $email\n Message:\n $message";


  if ($_POST['submit'] && $human == '4') {
    if (mail ($to, $subject, $body, $from)) {
      echo '
Your message has been sent!
';
    } else {
      echo '
Something went wrong, go back and try again!
';
    }
  } else if ($_POST['submit'] && $human != '4') {
    echo '
You answered the anti-spam question incorrectly!
';
  }


  if ($_POST['submit']) {
    if ($name != '' && $email != '') {
      if ($human == '4') {
        if (mail ($to, $subject, $body, $from)) {
          echo '
Your message has been sent!
';
        } else {
          echo '
Something went wrong, go back and try again!
';
        }
      } else if ($_POST['submit'] && $human != '4') {

        echo '
You answered the anti-spam question incorrectly!
';
      }
    } else {
      echo '
You need to fill in all required fields!!
';
    }
  }
?>

But constantly getting these errors:

Notice: Undefined index: name in C:\xampp\htdocs\bet4info\Index.php on
line 19

Notice: Undefined index: email in C:\xampp\htdocs\bet4info\Index.php
on line 20

Notice: Undefined index: message in C:\xampp\htdocs\bet4info\Index.php
on line 21

Notice: Undefined index: human in C:\xampp\htdocs\bet4info\Index.php
on line 25

Notice: Undefined index: submit in C:\xampp\htdocs\bet4info\Index.php
on line 29

Notice: Undefined index: submit in C:\xampp\htdocs\bet4info\Index.php
on line 35

Notice: Undefined index: submit in C:\xampp\htdocs\bet4info\Index.php
on line 38

Why is it so?

Answer

Instead of

$name = $_POST['name'];
if ($_POST['submit'] && $human == '4') {   

use

$name = isset($_POST['name']) ? $_POST['name'] : '';
if (isset($_POST['submit']) && $human == '4') {