php生成验证码

session_start();
getCode(4,60,20);
function getCode($num,$w,$h) {
 $code = "";
 for ($i = 0; $i < $num; $i++) {
 $code .= rand(0, 9);
 }
 //4位验证码也可以用rand(1000,9999)直接生成
 //将生成的验证码写入session，备验证时用
 $_SESSION["helloweba_num"] = $code;
 //创建图片，定义颜色值
 header("Content-type: image/PNG");
 $im = imagecreate($w, $h);
 $black = imagecolorallocate($im, 0, 0, 0);
 $gray = imagecolorallocate($im, 200, 200, 200);
 $bgcolor = imagecolorallocate($im, 255, 255, 255);
 //填充背景
 imagefill($im, 0, 0, $gray);
 //画边框
 imagerectangle($im, 0, 0, $w-1, $h-1, $black);
 //随机绘制两条虚线，起干扰作用
 $style = array ($black,$black,$black,$black,$black,
 $gray,$gray,$gray,$gray,$gray
 );
 imagesetstyle($im, $style);
 $y1 = rand(0, $h);
 $y2 = rand(0, $h);
 $y3 = rand(0, $h);
 $y4 = rand(0, $h);
 imageline($im, 0, $y1, $w, $y3, IMG_COLOR_STYLED);
 imageline($im, 0, $y2, $w, $y4, IMG_COLOR_STYLED);
 //在画布上随机生成大量黑点，起干扰作用;
 for ($i = 0; $i < 80; $i++) {
 imagesetpixel($im, rand(0, $w), rand(0, $h), $black);
 }
 //将数字随机显示在画布上,字符的水平间距和位置都按一定波动范围随机生成
 $strx = rand(3, 8);
 for ($i = 0; $i < $num; $i++) {
 $strpos = rand(1, 6);
 imagestring($im, 5, $strx, $strpos, substr($code, $i, 1), $black);
 $strx += rand(8, 12);
 }
 imagepng($im);//输出图片
 imagedestroy($im);//释放图片所占内存
}