convert png to jpg, compress 100 and replace file with jpg extension

function convert_file_to_jpg($file) {
    $image = imagecreatefrompng($file);
    $bg = imagecreatetruecolor(imagesx($image), imagesy($image));
    imagefill($bg, 0, 0, imagecolorallocate($bg, 255, 255, 255));
    imagealphablending($bg, TRUE);
    imagecopy($bg, $image, 0, 0, 0, 0, imagesx($image), imagesy($image));
    imagedestroy($image);
    $quality = 100; // 0 = worst / smaller file, 100 = better / bigger file 
    $file = str_replace('.png', '.jpg', $file);
    imagejpeg($bg, $file, $quality);
    imagedestroy($bg);
    return $file;
}