Why is it recommended to reference array keys in $_POST variables with quotes to prevent unexpected results in PHP scripts?

Referencing array keys in $_POST variables with quotes is recommended to prevent unexpected results in PHP scripts because if the key is not found in the $_POST array, PHP will automatically create a constant with the same name as the key. This can lead to unintended behavior or security vulnerabilities in the script. By using quotes around the array key, PHP will treat it as a string literal and not create a constant if the key is not found.

// Incorrect way of accessing array key in $_POST
$value = $_POST[key];

// Correct way of accessing array key in $_POST
$value = $_POST['key'];