Using reference to nonexistent value sets variable to NULL?

Question

When passing a non-existent value by reference, PHP creates the value and sets it to NULL. I noticed it when memory increases were occurring while checking empty values in some functions. Take the following function:

function v(&$v,$d=NULL){return isset($v)?$v:$d;}
$bar = v($foo, $default);

This would be shorthand for:

if(isset($foo))
{
    $bar = $foo;
}
else
{
    $bar = $default;
}

However, when passing non-existent variables PHP creates them. In the case of variables - they are removed as soon as the method/function ends - but for checking super global arrays like $_GET or $_POST the array element is never removed causing extra memory usage.

$request_with = v($_SERVER['HTTP_X_REQUESTED_WITH']);

Can anyone explain why this happens and if it is a PHP todo fix or a feature for some other crazy use of values?

Answer 1

XeonCross' function v is a shorthand for the often used:

$val= isset($arr['elm']) ? $arr['elm'] : 'default'

to avoid the dreaded 'Undefined index: elm' notice. A nice helper function would be:

function ifset(&$v1, $v2 = null) {
    return isset($v1) ? $v1 : $v2;
}

as Xeoncross suggested, so you could write the much nicer

$val = ifset($arr['elm'],'default') 

however, this has a lot of interesting (?) quirks in our beloved "language" that we call PHP:

inside the function ifset, $v1 seems UNSET, so it correctly returns the value $v2 and you might conclude that ifset works ok. But afterwards $arr['elm'] is silently set to NULL. So consider the following:

    function wtf(&$v) {
       if (isset($v))
           echo "It is set";
       else
           echo "It is NOT set";
    }
    $p=[];
    wtf($p['notexist']);   => It is NOT set
    $p;                    => [ 'notexist' => NULL ]

But this is another delusion, as the isset() function returns false for NULL values as well:

$x=NULL;
isset($x)   => false... huh??

Did we expect this? well.. it is in the documentation, so this is by design as well. Welcome to the wonderful world of php.