//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

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

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);

}