php如何用GD库使本地19201080图片从上到下渐变透明

以下是一个示例代码，可以使用GD库在本地1920*1080图片从上到下渐变透明：

<?php
// 创建一个1920*1080的图片
$image = imagecreatetruecolor(1920, 1080);

// 创建一个渐变透明的颜色
$transparent = imagecolorallocatealpha($image, 0, 0, 0, 127);

// 循环遍历每一行像素，从上到下逐渐变为透明
for ($y = 0; $y < 1080; $y++) {
    // 计算当前行的透明度
    $alpha = 127 - (127 / 1080) * $y;

    // 设置当前行的颜色为渐变透明的颜色
    imagesetstyle($image, array($transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, $transparent, 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