PHP 圖像處理完整速查：縮放、裁切、旋轉、翻轉、透明、銳化、水印

環境準備

# Windows php.ini
extension=php_gd2.dll

# Linux (Debian/Ubuntu)
sudo apt-get install php-gd
# CentOS/RHEL
sudo yum install php-gd

核心函式速查表

實用片段集錦

1. 等比縮放並白底置中（通用函式）

function thumbFit(string $src, string $dst, int $maxW, int $maxH, string $bgColor = '#FFFFFF'): bool {
    [$w, $h, $type] = getimagesize($src);
    $create = ['image/jpeg'=>'imagecreatefromjpeg','image/png'=>'imagecreatefrompng','image/gif'=>'imagecreatefromgif'][$type ?? ''];
    if (!$create) return false;
    $im = $create($src);

    $scale = min($maxW / $w, $maxH / $h);
    $nW = (int)round($w * $scale);
    $nH = (int)round($h * $scale);

    $canvas = imagecreatetruecolor($maxW, $maxH);
    $rgb = sscanf($bgColor, '#%02x%02x%02x');
    $bg = imagecolorallocate($canvas, ...$rgb);
    imagefill($canvas, 0, 0, $bg);

    imagecopyresampled($canvas, $im, ($maxW-$nW)/2, ($maxH-$nH)/2, 0, 0, $nW, $nH, $w, $h);

    $out = ['image/jpeg'=>'imagejpeg','image/png'=>'imagepng','image/gif'=>'imagegif'][$type];
    $out($canvas, $dst, 90);
    imagedestroy($im); imagedestroy($canvas);
    return true;
}

2. 裁切指定區域

function crop(string $src, string $dst, int $x, int $y, int $w, int $h): bool {
    $im = imagecreatefromjpeg($src); // 簡化：假設 JPEG
    $cut = imagecreatetruecolor($w, $h);
    imagecopyresampled($cut, $im, 0, 0, $x, $y, $w, $h, $w, $h);
    imagejpeg($cut, $dst, 90);
    imagedestroy($im); imagedestroy($cut);
    return true;
}

3. 旋轉 45 度（背景白）

$im = imagecreatefromjpeg('photo.jpg');
$rotated = imagerotate($im, 45, imagecolorallocate($im, 255, 255, 255));
imagejpeg($rotated, 'rotated.jpg', 90);
imagedestroy($im); imagedestroy($rotated);

4. 水平/垂直翻轉

function flipHorizontal(string $src, string $dst): void {
    $im = imagecreatefromjpeg($src);
    $w = imagesx($im); $h = imagesy($im);
    $out = imagecreatetruecolor($w, $h);
    for ($x = 0; $x < $w; $x++) {
        imagecopy($out, $im, $w - $x - 1, 0, $x, 0, 1, $h);
    }
    imagejpeg($out, $dst); imagedestroy($im); imagedestroy($out);
}
function flipVertical(string $src, string $dst): void {
    $im = imagecreatefromjpeg($src);
    $w = imagesx($im); $h = imagesy($im);
    $out = imagecreatetruecolor($w, $h);
    for ($y = 0; $y < $h; $y++) {
        imagecopy($out, $im, 0, $h - $y - 1, 0, $y, $w, 1);
    }
    imagejpeg($out, $dst); imagedestroy($im); imagedestroy($out);
}

5. PNG 透明保留縮放

function resizePngKeepAlpha(string $src, string $dst, int $w, int $h): bool {
    $im = imagecreatefrompng($src);
    imagesavealpha($im, true);
    $out = imagecreatetruecolor($w, $h);
    imagealphablending($out, false);
    imagesavealpha($out, true);
    imagecopyresampled($out, $im, 0, 0, 0, 0, $w, $h, imagesx($im), imagesy($im));
    imagepng($out, $dst);
    imagedestroy($im); imagedestroy($out);
    return true;
}

6. 簡易銳化（3x3 卷積核）

function sharpen(string $src, string $dst, float $amount = 1.0): bool {
    $im = imagecreatefromjpeg($src);
    $w = imagesx($im); $h = imagesy($im);
    $out = imagecreatetruecolor($w, $h);

    // 3x3 銳化核
    $kernel = [
        [0, -1, 0],
        [-1, 5, -1],
        [0, -1, 0]
    ];
    $div = array_sum(array_map('array_sum', $kernel));
    $offset = 0;

    if (function_exists('imageconvolution')) {
        imageconvolution($im, $kernel, $div, $offset);
        imagejpeg($im, $dst, 90);
    } else {
        // 手動像素級實作（略，效能極差）
    }
    imagedestroy($im); imagedestroy($out);
    return true;
}

7. 文字水印（中文支援）

function textWatermark(string $src, string $dst, string $text, string $font, int $size, array $rgb, int $x, int $y): bool {
    $im = imagecreatefromjpeg($src);
    $color = imagecolorallocate($im, ...$rgb);
    imagettftext($im, $size, 0, $x, $y, $color, $font, $text);
    imagejpeg($im, $dst, 90);
    imagedestroy($im);
    return true;
}

8. 圖片水印（支援 PNG 透明）

function imageWatermark(string $src, string $dst, string $wm, int $pos = 2, int $alpha = 100): bool {
    $dstIm = imagecreatefromjpeg($src);
    $wmIm = imagecreatefrompng($wm);
    imagealphablending($dstIm, true);
    imagesavealpha($dstIm, true);

    $dw = imagesx($dstIm); $dh = imagesy($dstIm);
    $ww = imagesx($wmIm); $wh = imagesy($wmIm);

    $positions = [
        0 => [0, 0],
        1 => [$dw-$ww, 0],
        2 => [$dw-$ww, $dh-$wh],
        3 => [0, $dh-$wh],
    ];
    [$px, $py] = $positions[$pos] ?? $positions[2];

    if ($alpha >= 100) {
        imagecopy($dstIm, $wmIm, $px, $py, 0, 0, $ww, $wh);
    } else {
        imagecopymerge($dstIm, $wmIm, $px, $py, 0, 0, $ww, $wh, $alpha);
    }
    imagejpeg($dstIm, $dst, 90);
    imagedestroy($dstIm); imagedestroy($wmIm);
    return true;
}

最佳實務建議

1. 記憶體管理：大圖處理前 ini_set('memory_limit', '512M')，用完立即 imagedestroy()。
2. 錯誤處理：每個 GD 函式回傳值檢查，拋出異常或記錄日誌。
3. 安全性：上傳檔案請用 getimagesize() 驗證真實類型，禁止直接執行上傳目錄腳本。
4. 現代化：新專案請直接 composer require intervention/image，享受鏈式 API、單元測試、Laravel/Symfony 整合。