PHP draws pie chart (imagefilledarc method)

$array = array("Beijing"=>1925,"Shanghai"=>2016,"Guangzhou"=>1256,"Shenzhen"=> 980);
$arr_key = array_keys($array);
$color = array();
$im = imagecreatetruecolor(300,300);
for($i=1;$i<=count($array);$i++ ){
 $color[] = imagecolorallocate($im,rand(0,255),rand(0,255),rand(0,255));
}
//Create a pie chart with multiple sectors consisting of
$a1=rand(0,360);
$sum = array_sum($array);
for($j=0;$j $a2 = $a1 + $arr_key[$j]/$sum*360;
 imagefilledarc($im,150,150,180,80,$a1,$a2,$color[$j],IMG_ARC_PIE);
 $a1 = $a2;
} // bbs .it-home.org
//Output image
header("content-type: image/png");
imagepng($im);
 //Close
imagedestroy($im);
?>