如何利用PHP函数实现图像处理？

$img = imagecreatefromjpeg('original.jpg');

$newWidth = 200;
$newHeight = 150;
$newImage = imagecreatetruecolor($newWidth, $newHeight);
imagecopyresized($newImage, $img, 0, 0, 0, 0, $newWidth, $newHeight, imagesx($img), imagesy($img));

$newWidth = 200;
$newHeight = 150;
$srcX = 50;
$srcY = 50;
$newImage = imagecreatetruecolor($newWidth, $newHeight);
imagecopy($newImage, $img, 0, 0, $srcX, $srcY, $newWidth, $newHeight);

$degree = 45;
$newImage = imagerotate($img, $degree, 0);

$fontSize = 20;
$textAngle = 0;
$textX = 10;
$textY = 10;
$textColor = imagecolorallocate($newImage, 255, 255, 255); // 水印文字颜色
$fontPath = 'font.ttf'; // 字体文件路径
imagefttext($newImage, $fontSize, $textAngle, $textX, $textY, $textColor, $fontPath, 'Watermark');

$srcX = 0;
$srcY = 0;
$dstX = 0;
$dstY = 0;
$dstWidth = imagesx($newImage);
$dstHeight = imagesy($newImage);
imagecopy($img, $newImage, $dstX, $dstY, $srcX, $srcY, $dstWidth, $dstHeight);

$outputPath = 'output.jpg';
imagejpeg($newImage, $outputPath);

header('Content-Type: image/jpeg');
imagejpeg($newImage);