php 逆向生成图片

这个先将PNG图片转为字符串 问怎么可以把字符串转为PNG

<?php

    $filename = "example.png";
    $image = imagecreatefrompng($filename);
    $str = "";
    if ($image != null)
    {
        $width = imagesx($image);
        $height = imagesy($image);
        $colors = array();
        for ($y = 0; $y < $height; $y++)
        {
            $y_array = array();
            for ($x = 0; $x < $width; $x++)
            {
                $rgb = imagecolorat($image, $x, $y);
                $r = ($rgb >> 16) & 0xFF;
                $g = ($rgb >> 8) & 0xFF;
                $b = $rgb & 0xFF;
                $x_array = array($r, $g, $b);
                $y_array[] = $x_array;
            }
            $colors[] = $y_array;
        }

        foreach ($colors as $width)
        {
            foreach ($width as $height)
            {
                $str .= chr($height[0]) . chr($height[1]) . chr($height[2]) . "\x00";
            }
        }
    }
    // var_dump($str);
?>