imagepolygon - Generate strange output

imagepolygon - Generate strange output

When try to run this code, a strange output happen.
<?php
// cria uma imagem em branco
$image = imagecreate(400, 300);
// prenche a cor do fundo
$bg = imagecolorallocate($image, 0, 0, 0);
// escolhe uma cor para o poligono
$col_poly = imagecolorallocate($image, 255, 255, 255);
// desenha o poligono
imagepolygon($image,
array (
0, 0,
100, 200,
300, 200
),
3,
$col_poly);
// envia a imagem
header("Content-type: image/png");
imagepng($image);
?>
The output is:
Warning: Cannot modify header information - headers already sent by
(output started at /var/www/drawimage/index.php:2) in
/var/www/drawimage/index.php on line 23
what should be the picture looks like this:
‰PNG
IHDR,×ywPLTEÿÿÿ¥ÙŸÝ1IDAThí•;ŽÜ0D5끡À€Ç™C±¡à#è(lt;åÌýùivWeX»+'Púa§8ÑYxä7üä'lt;ò-ÒÈKhdþÅ#3¦Ä#EÓwy²È?ñÈû‡ŒðÊÈôb'0}á'yMH#ïÏáò~V™yû¥òžV™ç:g\Ö+Nlt;8dmE*³¬2qšXeâñGDamp;ú¿ÌŠÊ„÷©ÌŠÊ¬¥He¶ï
ÙFŸSf¾p¶iá"ÙN™ý‚Ù;QÊì§L\œ21{‚'í¹ð¥LØž"2;B)³W¢"9¾Bä|F™¹xA9f…Qæ@eÎëMlt;pälD(sŒ2q1ÊÄê
DŽ÷Â#„2áxÊœ¡ÌYˆPæú'kìqeæfáæš\™Á•¹/7ñH@'»¬ÌàÊÄs+••¹VÀÊ„k+s#°2wX™ìD²¡G•™Õ¥™lNPe2Uamp;¿ÚÄ#Cò6
29‚*ï%ªL쬍H¶^xTamp;dkP™•ÉË€ÊýBŠ'Ç"™»›nŠ)Á")L™òb)»@Ê"¦LÌ7˜2ÑØuÅnáH™Pì
eJR¦¬)Sµ©Qf6·jªA"©D™úZ©›ÊÔ¢L,·ˆ2ÑÙ+'j¿ð L¨ö€25(S"iº&lsqauo;&lsqauo;4ãî+Ó
¾2Í„øÊ4ˆ¯L{©‰G‚‡´=\eZÄWamp;Ö¾2 â+#ÍÉÂ#®2¡9q•iW™¶†«ŒÒ\DvOñ"QæÃSFA
/code/pre
pwhat is happening?/p