ASCII art circle

<?php

$radius=12;
$w = 3;
for ($y = -$radius; $y <= $radius; $y++) {
     for ($x = -$radius; $x <= $radius; $x++) {
         $edge = ($x*$x + $y*$y) /  $radius - $radius;
         if ($edge > - $w*4/3 && $edge < 1) {
              echo "**";
          }else{
              echo "__";
          }
     }
    echo "</br>";
}