<?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 '你沒有安裝gd擴展';
?>

如果有返回信息就正確可以常用使用了
例1

復制代碼代碼如下:

<?php
/**
 * vCode(m,n,x,y) m個數字  顯示大小為n   邊寬x   邊高y
 * 自己改寫記錄session $code
 */
session_start(); 
vCode(4, 15); //4個數字，顯示大小為15
function vCode($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)];
 } 
 // 畫圖像
 $im = imagecreatetruecolor($width, $height); 
 // 定義要用到的顏色
 $back_color = imagecolorallocate($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)); 
 // 畫背景
 imagefilledrectangle($im, 0, 0, $width, $height, $back_color); 
 // 畫邊框
 imagerectangle($im, 0, 0, $width-1, $height-1, $boer_color); 
 // 畫干擾線
 for($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), $font_color);
 } 
 // 畫干擾點
 for($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), $font_color);
 } 
 // 畫驗證碼
 @imagefttext($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，結合session和GD庫擴展開發(fā)的一個生成驗證碼的例子（w3c推薦），可以很方便的用于項目中。而且樣式美觀

復制代碼代碼如下:

//首先開啟session
session_start();
//定義前臺顯示驗證碼長&寬
$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";
//定義要生成驗證碼的字符串
$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
//設置session，做驗證
$_SESSION['6_letters_code'] = $code;
function hexrgb ($hexstr)
{
  $int = hexdec($hexstr);
  return array("red" => 0xFF & ($int >> 0x10),
               "green" => 0xFF & ($int >> 0x8),
               "blue" => 0xFF & $int);
}