i use imagick, you can see there working flow https://aitextured.com/image_converter/ :

function convertRasterWithImagick(string $src, string $to, array $opts): string {
    $im = new Imagick();
    $im->readImage($src);
    $to = strtolower($to);

    if ($to === 'jpg' || $to === 'jpeg') {
        $quality = (int)($opts['quality'] ?? 90);
        $bgHex   = (string)($opts['bg'] ?? '#ffffff');
        if ($im->getImageAlphaChannel()) {
            [$r,$g,$b] = parseHexColor($bgHex);
            $canvas = new Imagick();
            $canvas->newImage($im->getImageWidth(), $im->getImageHeight(), new ImagickPixel("rgb($r,$g,$b)"));
            $canvas->compositeImage($im, Imagick::COMPOSITE_OVER, 0, 0);
            $im->destroy();
            $im = $canvas;
        }
        $im->setImageFormat('jpeg');
        $im->setImageCompression(Imagick::COMPRESSION_JPEG);
        $im->setImageCompressionQuality($quality);
        $tmp = tempnam(sys_get_temp_dir(), 'conv_').'.jpg';
        $im->writeImage($tmp);
        $im->destroy();
        return $tmp;
    }

    if ($to === 'png') {
        $im->setImageFormat('png');
        $im->setImageCompression(Imagick::COMPRESSION_ZIP);
        $im->setImageCompressionQuality(0);
        $tmp = tempnam(sys_get_temp_dir(), 'conv_').'.png';
        $im->writeImage($tmp);
        $im->destroy();
        return $tmp;
    }

    $im->destroy();
    throw new RuntimeException('Unsupported target format for raster: '.$to);
}