imagesetstyle() sets the style to be used by all line drawing functions when drawing with the special color .
Here goes a example of drawing a dashed-line circle.enjoy!
<?php
header("Content-type: image/jpeg");
$im = imagecreate(100,100);
$b = imagecolorallocate ($im, 0, 0, 0);
$w = imagecolorallocate ($im, 255, 255, 255);
$style = array ($b,$b,$b,$b,$b,$w,$w,$w,$w,$w);
imagesetstyle ($im, $style);
imagearc($im,50,50,100,100,0,360,IMG_COLOR_STYLED);
imagejpeg($im);
imagedestroy($im);
?>