PHP 8.0 how to append strings to unknown SESSION variables in a clean concise way without Warnings

[MHCreations]

Description

PHP version 8.0.15 I have empty error logs because I have notices turned off and I code to the best of my abilities and to best practise as far as practical.

The following code:

<?php
    session_start();
    $_SESSION['message'] .= "Sorry; That file or location can not be found.";   

Resulted in this output:

PHP Warning: Undefined array key "message" in /index.php on line 29

But I expected this output instead:

String is generated without error. 

The issue is that $_SESSION is an imported value; I can't force set the value as if a local variable; for fear of overwriting data in the session.

A workaround is

if(array_key_exists('message', $_SESSION)){
    $_SESSION['message'] .= "Sorry; That file or location can not be found.";
}
else {
    $_SESSION['message'] = "Sorry; That file or location can not be found.";
}

But look at that -- it's 5 extra lines and is a clear failure of DRY programming. With some shorthand encasing it can be reduced down to

$_SESSION['message'] = (array_key_exists('message', $_SESSION)?$_SESSION['message']:"") 
                       ."Sorry; That file or location can not be found.";

Which is verbose, hard to read but still on one line, the additional code doesn't DO anything. Apart from hiding the warning message.

Obviously turning off warning messages is not a good idea, and I am left with many, many websites using many thousands of instances of $_SESSION data concatenation that now generate verbose and pointless errors; causing genuine errors harder to find.

So; Can using the array key exist WARNING be relegated to Notice for when string concatenation occurs or for when acted on superglobals (such as $_SESSION) which come into the PHP scope as data from outside and thus can't be set within the script itself.

 Thank you. 

[kamil-tekiela]

What's wrong with this?

<?php
    session_start();
    $_SESSION['message'] ??= '';
    $_SESSION['message'] .= "Sorry; That file or location can not be found.";   

[MHCreations]

That is a possible workaround that I hadn't considered.
It does still smell of verbose code the need to explicitly set an empty value in order to append to it. But yes, thanks, that's one workaround.

[iluuu1994]

Or $_SESSION['message'] = ($_SESSION['message'] ?? '') . 'Sorry; That file or location can not be found.'; if you want a one-liner.