<?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);?>
注！关于验证码无法显示，首先判断GD库是否开启；
默认配置下，php输出是先到输出缓冲区（output_buffering），只要数据还没有真正发送到浏览器（严格来说是tcp buffer），那么还是有机会清空先前的缓冲区里面的数据，使用内置的ob_clean函数即可。
php 验证码 图像存在错误无法显示
原文地址：http://www.cnblogs.com/fan-bk/p/7660615.html