Fill the transparent part of transparent PNG with a solid color.

set_bg_color.php

<?php

$fpath = $argv[1];

$o = new Bg();
$o->setBgColor($fpath);


class Bg
{
    public function setBgColor($file_path, $rgb = [255, 255, 255])
    {
        try
        {
            if (!file_exists($file_path))
            {
                throw new Exception(sprintf('target file does not exists. >>> %s', $file_path));
            }

            list($width, $height, $type, $attr) = getimagesize($file_path);
            $image = imagecreatefrompng($file_path);

            // 背景画像生成
            $bg = imagecreatetruecolor($width, $height);
            $c = imagecolorallocate($bg, $rgb[0], $rgb[1], $rgb[2]);
            imagefilledrectangle($bg, 0, 0, $width, $height, $c);

            // 画像を合成
            imagecopy($bg, $image, 0, 0, 0, 0, $width, $height);

            // 出力
            imagepng($bg, $file_path);

            imagedestroy($bg);
            imagedestroy($image);
        }
        catch (Exception $e)
        {
            echo $e->getMessage();
        }
    }
}