Does it matter if I use
$_SERVER["DOCUMENT_ROOT"] rather than
$_SERVER['DOCUMENT_ROOT']
In the code I am struggling with it uses double quotes rather than single quotes
as you show here,
No. There's no practical difference between single and double quotes here.
The only difference between single and double quotes is that double quotes allow the use of variables and escape sequences (single quoted strings only parse the \\ and \' escape sequences)
See:
https://www.php.net/manual/en/language.types.string.php