utf 8 - Convert Japanese characters to UTF-8 for php json

i have japanese string in (utf-8 encoded) database '利用規約' passed php's json_encode function , receive warning:

php warning: json_encode(): invalid utf-8 sequence in argument

is there way convert valid utf-8 in order avoid this? if db utf-8 why php have problem string?

update:

turns out error in fact due problem sprintf , multibyte characters couple lines earlier.

utf8_encode should work you.

if utf8_encode doesn't work, try mb_convert_encoding:

mb_convert_encoding($string,"utf-8","auto");

HR