PHP を使用して画像の解像度を調整し、形式を変換する方法

<?php
function resizeImage($sourceImage, $targetImage, $newWidth, $newHeight) {
    list($sourceWidth, $sourceHeight, $sourceType) = getimagesize($sourceImage);
    $sourceType = image_type_to_mime_type($sourceType);
    $sourceGdImage = false;
    switch ($sourceType) {
        case 'image/jpeg':
            $sourceGdImage = imagecreatefromjpeg($sourceImage);
            break;
        case 'image/png':
            $sourceGdImage = imagecreatefrompng($sourceImage);
            break;
        case 'image/gif':
            $sourceGdImage = imagecreatefromgif($sourceImage);
            break;
        default:
            return false;
    }
    if (!$sourceGdImage) {
        return false;
    }
    $targetGdImage = imagecreatetruecolor($newWidth, $newHeight);
    imagecopyresampled($targetGdImage, $sourceGdImage, 0, 0, 0, 0, $newWidth, $newHeight, $sourceWidth, $sourceHeight);
    imagejpeg($targetGdImage, $targetImage);
    imagedestroy($sourceGdImage);
    imagedestroy($targetGdImage);
    return true;
}

// 调用示例
$sourceImage = 'source.jpg';
$targetImage = 'target.jpg';
$newWidth = 800;
$newHeight = 600;
resizeImage($sourceImage, $targetImage, $newWidth, $newHeight);
?>

<?php
function convertImageFormat($sourceImage, $targetImage, $targetFormat) {
    list($sourceWidth, $sourceHeight, $sourceType) = getimagesize($sourceImage);
    $sourceGdImage = false;
    switch ($sourceType) {
        case IMAGETYPE_JPEG:
            $sourceGdImage = imagecreatefromjpeg($sourceImage);
            break;
        case IMAGETYPE_PNG:
            $sourceGdImage = imagecreatefrompng($sourceImage);
            break;
        case IMAGETYPE_GIF:
            $sourceGdImage = imagecreatefromgif($sourceImage);
            break;
        default:
            return false;
    }
    if (!$sourceGdImage) {
        return false;
    }
    $targetGdImage = imagecreatetruecolor($sourceWidth, $sourceHeight);
    imagecopy($targetGdImage, $sourceGdImage, 0, 0, 0, 0, $sourceWidth, $sourceHeight);
    switch ($targetFormat) {
        case 'jpeg':
            imagejpeg($targetGdImage, $targetImage);
            break;
        case 'png':
            imagepng($targetGdImage, $targetImage);
            break;
        case 'gif':
            imagegif($targetGdImage, $targetImage);
            break;
        default:
            return false;
    }
    imagedestroy($sourceGdImage);
    imagedestroy($targetGdImage);
    return true;
}

// 调用示例
$sourceImage = 'source.jpg';
$targetImage = 'target.png';
$targetFormat = 'png';
convertImageFormat($sourceImage, $targetImage, $targetFormat);
?>