How to fix Undefined Variable/Index Notice?

A common Notice when working with PHP can be:

Notice: Undefined variable: my_var in C:\xampp\htdocs\index.php on line 14

or

Notice: Undefined index: my_index C:\xampp\htdocs\index.php on line 24

Why does this happen?

You have some lines to set a Variable or get posted Data from a Form like this:

<?php
  $my_var = $_POST['myPostData'];
?>

Now PHP does not have $_POST[‘myPostData’] if there is no POST Data being sent to the Page. This is when these Errors come into Play.

How to fix “Undefined variable”?

<?php
  $my_var = ""; // Or $my_var = 0; for numbers to make sure the Variable is initialised
  //now use isset()
  $my_var = isset($_POST['myPostData']) ? $_POST['myPostData'] : "";
  //or empty()
  $my_var = !empty($_POST['myPostData']) ? $_POST['myPostData'] : "";
?>

How to fix “Undefined index”?

<?php
 //again we have two options here
 //via array_key_exists()
 $my_var = array_key_exists('my_index', $my_array) ? $my_array['my_index'] : "";
 //or via isset()
 $my_var = isset($my_array['my_index']) ? $my_array['my_index'] : "";
?>

Null Coalescing Operator

Since PHP 7 there is also the ?? (or Null Coalescing) Operator available:

<?php
  $my_var = $_POST['myPostData'] ?? "";
?>
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