最简单的php验证码

<?php
session_start();
// Settings: You can customize the captcha here
$image_width = 120;
$image_height = 40;
$characters_on_image = 4;
$font = 'c:/windows/fonts/arial.ttf';

// The characters that can be used in the CAPTCHA code.
// avoid confusing characters (l 1 and i for example)
$possible_letters = '23456789bcdfghjkmnpqrstvwxyzABCDEFGHJKMNPQRSTWXYZ';
$random_dots = 10;
$random_lines = 30;
$captcha_text_color = "0x142864";
$captcha_noice_color = "0x142864";

$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
$_SESSION['code'] = strtoupper($code);

function hexrgb($hexstr)
{
    $int = hexdec($hexstr);

    return array(
        "red" => 0xFF & ($int >> 0x10),
        "green" => 0xFF & ($int >> 0x8),
        "blue" => 0xFF & $int
    );
}
?>

　　

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