json_encode(): Invalid UTF-8 sequence in argument
I had a similar error which caused json_encode to return a null field whenever there was a hi-ascii character such as a curly apostrophe in a string, due to the wrong character set being returned in the query.
The solution was to make sure it comes as utf8 by adding:
mysql_set_charset('utf8');after the mysql connect statement.
Seems like the symbol was Å, but since data consists of surnames that shouldn't be public, only first letter was shown and it was done by just $lastname[0], which is wrong for multibyte strings and caused the whole hassle. Changed it to mb_substr($lastname, 0, 1) - works like a charm.
The problem is that this character is UTF8, but json_encode does not handle it correctly.To say more, there is a list of other characters (see Unicode characters list), that will trigger the same error, so stripping off this one (Å) will not correct an issue to the end.
What we have used is to convert these chars to html entities like this:
htmlentities( (string) $value, ENT_QUOTES, 'utf-8', FALSE);