What are the implications of missing server variables like $DOCUMENT_ROOT in PHP scripts and how can it affect the functionality of PHP applications?

Missing server variables like $DOCUMENT_ROOT in PHP scripts can affect the functionality of PHP applications, especially when the script relies on this variable to access files or directories. To solve this issue, you can use the $_SERVER superglobal array to access server variables like DOCUMENT_ROOT.

$document_root = $_SERVER['DOCUMENT_ROOT'];