php base64 encode appears unexpected character

I have a xml string in php look like this.

......

And I convert it to base64 form by

base64_encode()

However, the client say after decoding he found my xml looks like

.......

so return a error code to me.

I have already try use iconv to convert my xml str from utf-16 to utf-8, set notepad++'s encoding to utf-8 without bom.
but it's cant help.
Cant ask the client to change his code.