[PHP] Use mb_detect_encoding when available (#3952)

I've faced an issue where I want to serialize UTF-8 emojis and FlatBufferBuilder::is_utf8 would return false on them.
I was not able to add the corresponding hexadecimal code because I don't understand how the whole thing works but what i've done is using the `mb_detect_encoding` function which handles correctly all supposed UTF-8 characters.
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