/ is not a PCRE metacharacter, so although you're using it as the delimiter in your regular expression, preg_quote() won't escape it because it doesn't know you're using it as the delimiter. You'll have to pass it in as the second parameter, as the rest have said, in order for it to be escaped too:
preg_match_all("/[^\s]*".preg_quote($value, '/')."[^\s]*/iu", $row_search['content'], $final_matched);
