PHP 產生 Session ID 與驗證碼圖片完整實作

1. 產生唯一 Session ID create_sess_id()

結合微秒時間與隨機數，經雙重 MD5 雜湊後截取指定長度（預設 32 字元，最小 16）。

function create_sess_id($len = 32) {
    if (!is_numeric($len) || $len > 32 || $len < 16) return;

    list($usec, $sec) = explode(' ', microtime());
    $time = (float)$usec + (float)$sec;

    $rand = rand(100000, 999999);
    $rand = rand($rand, $time);
    mt_srand($rand);
    $rand = mt_rand();

    $sess = md5(md5($time) . md5($rand));
    return substr($sess, 0, $len);
}

2. 產生驗證碼 create_check_code()

直接從 Session ID 中截取片段（預設 4 字元，最大 6），轉大寫回傳。

function create_check_code($len = 4) {
    if (!is_numeric($len) || $len > 6 || $len < 1) return;
    return strtoupper(substr(create_sess_id(), 16, $len));
}

3. 繪製驗證碼圖片 create_check_image()

使用 GD 函式建立 65x22 px 圖片，灰底白字，加入 200 個隨機干擾像素。

function create_check_image($code) {
    $im = imagecreate(65, 22);
    $bg   = imagecolorallocate($im, 200, 200, 200);
    $fg   = imagecolorallocate($im, 255, 255, 255);
    imagefill($im, 68, 30, $bg);
    imagestring($im, 5, 8, 3, $code, $fg);

    for ($i = 0; $i < 200; $i++) {
        $color = imagecolorallocate($im, rand(0,255), rand(0,255), rand(0,255));
        imagesetpixel($im, rand()%70, rand()%30, $color);
    }

    header('Content-Type: image/png');
    imagepng($im);
    imagedestroy($im);
}

使用範例

<?php
// captcha.php —— 專門輸出驗證碼圖片
session_start();
require_once 'functions.php'; // 上述三個函數

$code = create_check_code(4);
$_SESSION['verify_code'] = $code;
create_check_image($code);
?>

<img src="captcha.php" alt="驗證碼" onclick="this.src='captcha.php?'+Math.random()">
<input name="captcha" placeholder="輸入圖片代碼">

進階版：可設定寬高、干擾線、字體大小的驗證碼

<?php
session_start();

$imgW = 80; $imgH = 20;
$pixelNum = 200; $useLine = true; $pixelMode = 2;

function make_seed() {
    list($usec, $sec) = explode(' ', microtime());
    return (float)$sec + ((float)$usec * 100000);
}
mt_srand(make_seed());
$authNum = mt_rand(100, 99999);
$_SESSION['verifycode'] = $authNum;

header('Content-Type: image/png');
$im = imagecreatetruecolor($imgW, $imgH);
$bg = imagecolorallocate($im, mt_rand(250,255), mt_rand(250,255), mt_rand(250,255));
imagefill($im, 0, 0, $bg);

$wordInfo = []; $totalW = 0;
for ($i = 0; $i < strlen($authNum); $i++) {
    $space = mt_rand(1,5);
    $font  = rand(3,5);
    $top   = (mt_rand(1,9)%2) ? 1 : 3;
    $wordInfo[$i] = [
        'char'   => $authNum[$i],
        'font'   => $font,
        'offset' => $top,
        'width'  => imagefontwidth($font) + $space,
        'height' => imagefontheight($font),
        'color'  => imagecolorallocate($im, mt_rand(0,50), mt_rand(0,150), mt_rand(0,200))
    ];
    $totalW += $wordInfo[$i]['width'];
    if ($i === 0) $totalH = $wordInfo[$i]['height'];
}

$ox = floor(($imgW - $totalW)/2);
$oy = floor(($imgH - $totalH)/2);

$w = 0;
foreach ($wordInfo as $v) {
    imagestring($im, $v['font'], $ox + $w, $v['offset'] + $oy, $v['char'], $v['color']);
    $w += $v['width'];
}

// 干擾點
if ($pixelMode == 1) {
    $c = imagecolorallocate($im, mt_rand(50,255), mt_rand(50,255), mt_rand(50,255));
    for ($i=0;$i<$pixelNum;$i++) imagesetpixel($im, mt_rand()%$imgW, mt_rand()%$imgH, $c);
} elseif ($pixelMode == 2) {
    for ($i=0;$i<=128;$i++) {
        $c = imagecolorallocate($im, rand(0,255), rand(0,255), rand(0,255));
        imagesetpixel($im, mt_rand(2,128), mt_rand(2,38), $c);
    }
}

imagepng($im);
imagedestroy($im);
?>

注意事項

• 驗證碼圖片必須在**獨立檔案**輸出，不能混雜 HTML/空白字元，否則圖片損壞。
• Session 驗證時需 session_start() 並比對 $_SESSION['verify_code']（大小写不敏感建議 strtoupper）。
• 現代專案建議使用 Gregwar/Captcha 或 mewebstudio/captcha 等成熟套件，支援更複雜干擾、字型、語音驗證碼等。