what does this do

[Talguy]

I'm trying to find the most common color in an image giving my application the ability to search by color. I was searching php.net for something that would tell be the rgb values for each pixel of the image which i found as imagecolorat(). A little ways down someone wrote a function that finds the most common color and then converts it into hexidecimal. but there is three lines of code that I do not understand.

here is all the code

function average($img) {
  $w = imagesx($img);
  $h = imagesy($img);
  $r = $g = $b = 0;
  for($y = 0; $y < $h; $y++) {
    for($x = 0; $x < $w; $x++) {
      $rgb = imagecolorat($img, $x, $y);
      $r += $rgb >> 16;
      $g += $rgb >> 8 & 255;
      $b += $rgb & 255;
    }
  }
  $pxls = $w * $h;
  $r = dechex(round($r / $pxls));
  $g = dechex(round($g / $pxls));
  $b = dechex(round($b / $pxls));
  if(strlen($r) < 2) {
    $r = 0 . $r;
  }
  if(strlen($g) < 2) {
    $g = 0 . $g;
  }
  if(strlen($b) < 2) {
    $b = 0 . $b;
  }
  return "#" . $r . $g . $b;
}

what I do not understand is this section what does the >> mean and where they use this &

      $r += $rgb >> 16;
      $g += $rgb >> 8 & 255;
      $b += $rgb & 255;

[pete_bisby]

The '>>' is a bitwise operation - it shifts the value of $rgb so many places to the right.

The imagecolorat function returns an integer value representing red, blue and green, each value between 0 and 255 (8 bits each). The bitwise operation will shift 16 bits to the right to get the red value, and 8 bits to the right to get the blue value.

The ampersand is a masking operation to show the hex values of the numbers. Have a look at http://uk2.php.net/manual/en/function.imagecolorat.php to see some examples of this.