php 验证码 图像存在错误无法显示

<?php

$height = 300;
$width = 300;
$im = imagecreatetruecolor($width, $height);
$white = imagecolorallocate ($im, 255, 255, 255);
$blue = imagecolorallocate ($im, 0, 0, 64);
imagefill($im, 0, 0, $blue);
imagestring($im, 10, 100, 120, ‘Hello,PHP‘, $white);
ob_clean(); //如果验证码输出不了，加上这一句就可以输出了
header (‘Content-type: image/png‘);
imagepng($im);imagedestroy($im);

?>

<?php
//无bug的国旗测试
//ob_clean();
header("Content-Type:image/jpeg");
$ing = imagecreatetruecolor(700,410);
//新建一个真彩色图像，返回值为一个图像标识符，背景默认为黑色，参数(x_size*y_size)
$red = imagecolorallocate($ing,255,0,0);//定义背景颜色
$yellow = imagecolorallocate($ing,237,231,32);//定义黄色
imagefill($ing,0,0,$red);//填充颜色，以坐标(0,0)开始起填充
//数组坐标，表示(x1,y1,x2,y2,x3,y3.....x11,y11);
$a = array(90,30,108,73,157,73,119,102,135,152,93,123,52,152,66,102,29,74,76,73,90,30);
imagefilledpolygon($ing,$a,10,$yellow);//画一个多边形：10表示顶点总数，$yellow表示填充色
$a1 = array(229,25,229,43,248,48,229,55,229,74,217,60,198,66,210,50,197,34,218,39,229,25);
imagefilledpolygon($ing,$a1,10,$yellow);
$a2 = array(227,108,227,127,245,134,228,139,227,157,215,143,196,149,208,132,196,117,215,122,227,108);
imagefilledpolygon($ing,$a2,10,$yellow);
$a3 = array(163,184,163,204,181,211,163,216,163,234,152,220,132,225,144,209,132,193,151,199,163,184);
imagefilledpolygon($ing,$a3,10,$yellow);
$a4 = array(65,209,65,228,84,235,65,240,65,259,54,245,33,249,46,233,34,217,53,224,68,209);
imagefilledpolygon($ing,$a4,10,$yellow);
imagejpeg($ing);
imagedestroy($ing);
?>