Yes it $ddfdfdf is empty, isset will return FALSE.

According to the following script :

<?php

$test = array ('a' => 'coooool','b' => "$tests");
if (isset($test['b']) ) {
echo "OK 1! \n";
}

$test = array ('a' => 'coooool','b' => $tests);
if ( isset($test['b']) ) {
echo "OK 2! \n";
}

$tests = "lol";

$test = array ('a' => 'coooool','b' => $tests);
if ( isset($test['b']) ) {
echo "OK 3! \n";
}

?>

That returns:

OK 1!
OK 3!

If you set an element of an array to a variable that is empty, isset will definatly return FALSE.

Excepted behaviour.

Note:

$test = ""; sets the variable test to an empty string. 
So isset() will return TRUE. Since you're passing "$var" and $var is empty, it will be set to an empty string. 

That explains why it returns TRUE with "$var" and not with only $var.

Thank you for your report.