电话号码用图片显示

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<?php
ob_start();
Header("Content-type: image/png");
$tel = $_GET['telnum'];
$im = imagecreate(80,21);
$di = ImageColorAllocate($im, 0,0,0);
$zi = ImageColorAllocate($im, 255,255,255);
for($i=0;$i<100;$i++){ImageSetPixel($im,rand(0,80),rand(0,21),$zi);}
imagestring($im,5,2,1,$tel,$zi);
ImagePng($im);
ImageDestroy($im);
?>