insert associative array to a unique variable

I wrote a associative array and wanna define a key to a unique variable:

<?php
$birthday = [
    'Kevin' => '20-10-1990',
    'Max' => '11-11-2007',
    'Nadia' => '02-03-2000'
];
$birthday['Nadia'] = $nadia;
echo 'Nadia's Birthday is: ' . $nadia;
?>

But there is warning:

**Warning: Undefined variable $nadia

**

and the Result is empty:

Nadia’s Birthday is:

why don’t recognize php this variable ? Or recognized the variable, but can’t read the value from Nadia?