how to get from form input absolute value php : I know it is abs() but why this one does not work ? in php or if its wrong how to make it right:

    <?php 
    echo "Number";
    echo "\x20"; 
    echo $_GET["luku"];
    echo "\x20"; 
    echo "absolute value is";
    echo "\x20"; 
    echo $_GET[abs("luku")]; 
    ?>
first 'get' is working but second one not.
Im just increasing my knowledge on php