php判断base64字节大小

/* 判断base64图片数据大小不超过50M */
$base64 = str_replace('data:image/jpeg;base64,', '', $data);
$base64 = str_replace('=', '', $base64);
$img_len = strlen($base64);
$file_size = $img_len - ($img_len / 8) * 2;
$file_size = number_format(($file_size / 1024), 2).'kb';
if ($file_size > (1024 * 1024 * 50)) {
    echo "image size: ".$file_size."<br/>";
    echo "image is too large";
    exit();
}
echo "Image size: ".$file_size."<br/>";