Want to have some fun with this function?  How about edge detection! (ported from http://bitecode.co.uk/2008/07/edge-detection-in-python/)

<?php
function edge($input, $output)
{
  $in_im = imageCreateFromJpeg($input);
  $gx = array(array(-1, 0, 1), array(-2, 0, 2), array(-1, 0, 1));
  $gy = array(array(-1, -2, -1), array(0, 0, 0), array(1, 2, 1));
  $x = imagesx($in_im);
  $y = imagesy($in_im);
  $out_im = imagecreatetruecolor($x, $y);
  $colors = array(255 => imagecolorallocate($out_im, 255, 255, 255));
  for ($row = 1; $row < $x; $row++)
  {
    for ($col = 1; $col < $y; $col++)
    {
      $eyedropper =imagecolorat($in_im, $x, $y);
      $color =imagecolorsforindex($in_im, $eyedropper);
      $pxval = ($color['red'] + $color['green'] + $color['blue']) / 3;
      $pixel_gx = $pixel_gy = 0;
      for ($i = -1; $i < 2; $i++)
      {
        for ($j = -1; $j < 2; $j++)
        {
          $eyedropper =imagecolorat($in_im, $row+$i, $col+$j);
          $color =imagecolorsforindex($in_im, $eyedropper);
          $val = ($color['red'] + $color['green'] + $color['blue']) / 3;
          $pixel_gx += $gx[$i+1][$j+1] * $val;
          $pixel_gy += $gy[$i+1][$j+1] * $val;
        }
      }
      $pixel = sqrt($pixel_gx * $pixel_gx + $pixel_gy * $pixel_gy);
      $pixel = abs(255 - (int)$pixel);
      if (!isset($colors[$pixel])) $colors[$pixel] = imagecolorallocate($out_im, $pixel, $pixel, $pixel);
      imageSetPixel($out_im, $row, $col, $colors[$pixel]);
    }
  }
  imagejpeg($out_im,$output, 75);
}

edge('input.jpg', 'output.jpg');

?>