我们先来检查一下自己的php是不是打开了gd库。
复制代码 代码如下:
<?php
if(extension_loaded('gd')) {
  echo '你可以使用gd<br>';
  foreach(gd_info() as $cate=>$value)
    echo "$cate: $value<br>";
}else
  echo '你没有安装gd扩展';
?>

如果有返回信息就正确可以常用使用了
例1
复制代码 代码如下:
<?php
/**
 * vCode(m,n,x,y) m个数字  显示大小为n   边宽x   边高y
 * 自己改写记录session $code
 */
session_start();
vCode(4, 15); //4个数字，显示大小为15
function vCode($num = 4, $size = 20, $width = 0, $height = 0) {
 !$width && $width = $num * $size * 4 / 5 + 5;
 !$height && $height = $size + 10;
 // 去掉了 0 1 O l 等
 $str = "23456789abcdefghijkmnpqrstuvwxyzABCDEFGHIJKLMNPQRSTUVW";
 $code = '';
 for ($i = 0; $i < $num; $i++) {
  $code .= $str[mt_rand(0, strlen($str)-1)];
 }
 // 画图像
 $im = imagecreatetruecolor($width, $height);
 // 定义要用到的颜色
 $back_color = imagecolorallocate($im, 235, 236, 237);
 $boer_color = imagecolorallocate($im, 118, 151, 199);
 $text_color = imagecolorallocate($im, mt_rand(0, 200), mt_rand(0, 120), mt_rand(0, 120));
 // 画背景
 imagefilledrectangle($im, 0, 0, $width, $height, $back_color);
 // 画边框
 imagerectangle($im, 0, 0, $width-1, $height-1, $boer_color);
 // 画干扰线
 for($i = 0;$i < 5;$i++) {
  $font_color = imagecolorallocate($im, mt_rand(0, 255), mt_rand(0, 255), mt_rand(0, 255));
  imagearc($im, mt_rand(- $width, $width), mt_rand(- $height, $height), mt_rand(30, $width * 2), mt_rand(20, $height * 2), mt_rand(0, 360), mt_rand(0, 360), $font_color);
 }
 // 画干扰点
 for($i = 0;$i < 50;$i++) {
  $font_color = imagecolorallocate($im, mt_rand(0, 255), mt_rand(0, 255), mt_rand(0, 255));
  imagesetpixel($im, mt_rand(0, $width), mt_rand(0, $height), $font_color);
 }
 // 画验证码
 @imagefttext($im, $size , 0, 5, $size + 3, $text_color, 'c:\WINDOWS\Fonts\simsun.ttc', $code);
 $_SESSION["VerifyCode"]=$code;
 header("Cache-Control: max-age=1, s-maxage=1, no-cache, must-revalidate");
 header("Content-type: image/png;charset=gb2312");
 imagepng($im);
 imagedestroy($im);
}
?>

例2
使用PHP，结合session和GD库扩展开发的一个生成验证码的例子（w3c推荐），可以很方便的用于项目中。而且样式美观
复制代码 代码如下:
//首先开启session
session_start();
//定义前台显示验证码长&宽
$image_width = 120;
$image_height = 40;
$characters_on_image = 6;
$font = './monofont.ttf';
//The characters that can be used in the CAPTCHA code.
//avoid confusing characters (l 1 and i for example)
$possible_letters = '23456789bcdfghjkmnpqrstvwxyz';
$random_dots = 10;
$random_lines = 30;
$captcha_text_color="0x142864";
$captcha_noice_color = "0x142864";
//定义要生成验证码的字符串
$code = '';
$i = 0;
while ($i < $characters_on_image) {
$code .= substr($possible_letters, mt_rand(0, strlen($possible_letters)-1), 1);
$i++;
}
$font_size = $image_height * 0.75;
$image = @imagecreate($image_width, $image_height);
/* setting the background, text and noise colours here */
$background_color = imagecolorallocate($image, 255, 255, 255);
$arr_text_color = hexrgb($captcha_text_color);
$text_color = imagecolorallocate($image, $arr_text_color['red'],
        $arr_text_color['green'], $arr_text_color['blue']);
$arr_noice_color = hexrgb($captcha_noice_color);
$image_noise_color = imagecolorallocate($image, $arr_noice_color['red'],
        $arr_noice_color['green'], $arr_noice_color['blue']);
/* generating the dots randomly in background */
for( $i=0; $i<$random_dots; $i++ ) {
imagefilledellipse($image, mt_rand(0,$image_width),
 mt_rand(0,$image_height), 2, 3, $image_noise_color);
}
/* generating lines randomly in background of image */
for( $i=0; $i<$random_lines; $i++ ) {
imageline($image, mt_rand(0,$image_width), mt_rand(0,$image_height),
 mt_rand(0,$image_width), mt_rand(0,$image_height), $image_noise_color);
}
/* create a text box and add 6 letters code in it */
$textbox = imagettfbbox($font_size, 0, $font, $code);
$x = ($image_width - $textbox[4])/2;
$y = ($image_height - $textbox[5])/2;
imagettftext($image, $font_size, 0, $x, $y, $text_color, $font , $code);
/* Show captcha image in the page html page */
header('Content-Type: image/jpeg');// defining the image type to be shown in browser widow
imagejpeg($image);//showing the image
imagedestroy($image);//destroying the image instance
//设置session，做验证
$_SESSION['6_letters_code'] = $code;
function hexrgb ($hexstr)
{
  $int = hexdec($hexstr);
  return array("red" => 0xFF & ($int 0x10),
               "green" => 0xFF & ($int 0x8),
               "blue" => 0xFF & $int);
}

个人推荐推荐第二个生成验证码程序代码，各位同学可尝试参考对比哦，最后一个是W3C标准生成的也是利用了php gd库。