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I would like to make sure that everything I know about UTF-8 is correct. I have been trying to use UTF-8 for a while now but I keep stumbling across more and more bugs and other weird things that make it seem almost impossible to have a 100% UTF-8 site. There is always a gotcha somewhere that I seem to miss. Perhaps someone here can correct my list or OK it so I don't miss anything important.


Every site has to store there data somewhere. No matter what your PHP settings are you must also configure the DB. If you can't access the config files then make sure to "SET NAMES 'utf8'" as soon as you connect. Also, make sure to use utf8_ unicode_ ci on all of your tables. This assumes MySQL for a database, you will have to change for others.


I do a LOT of regex that is more complex than your average search-replace. I have to remember to use the "/u" modifier so that PCRE doesn't corrupt my strings. Yet, even then there are still problems apparently.

String Functions

All of the default string functions (strlen(), strpos(), etc.) should be replaced with Multibyte String Functions that look at the character instead of the byte.

Headers You should make sure that your server is returning the correct header for the browser to know what charset you are trying to use (just like you must tell MySQL).

header('Content-Type: text/html; charset=utf-8');

It is also a good idea to put the correct < meta > tag in the page head. Though the actual header will override this should they differ.

<meta http-equiv="Content-Type" content="text/html;charset=utf-8">


Do I need to convert everything that I receive from the user agent (HTML form's & URI) to UTF-8 when the page loads or if I can just leave the strings/values as they are and still run them through these functions without a problem?

If I do need to convert everything to UTF-8 - then what steps should I take? mb_detect_encoding seems to be built for this but I keep seeing people complain that it doesn't always work. mb_check_encoding also seems to have a problem telling a good UTF-8 string from a malformed one.

Does PHP store strings in memory differently depending on what encoding it is using (like file types) or is it still stored like a regular sting with some of the chars being interpreted differently (like & amp; vs & in HTML). chazomaticus answers this question:

In PHP (up to PHP5, anyway), strings are just sequences of bytes. There is no implied or explicit character set associated with them; that's something the programmer must keep track of.

If a give a non-UTF-8 string to a mb_* function will it ever cause a problem?

If a UTF string is improperly encoded will something go wrong (like a parsing error in regex?) or will it just mark an entity as bad (html)? Is there ever a chance that improperly encoded strings will result in function returning FALSE because the string is bad?

I have heard that you should mark you forms as UTF-8 also (accept-charset="UTF-8") but I am not sure what the benefit is..?

Was UTF-16 written to address a limit in UTF-8? Like did UTF-8 run out of space for characters? (Y2(UTF)k?)


Here are are a couple of the custom PHP functions I have found but I haven't any way to verify that they actually work. Perhaps someone has an example which I can use. First is convertToUTF8() and then seems_utf8 from wordpress.

function seems_utf8($str) {
    $length = strlen($str);
    for ($i=0; $i < $length; $i++) {
        $c = ord($str[$i]);
        if ($c < 0x80) $n = 0; # 0bbbbbbb
        elseif (($c & 0xE0) == 0xC0) $n=1; # 110bbbbb
        elseif (($c & 0xF0) == 0xE0) $n=2; # 1110bbbb
        elseif (($c & 0xF8) == 0xF0) $n=3; # 11110bbb
        elseif (($c & 0xFC) == 0xF8) $n=4; # 111110bb
        elseif (($c & 0xFE) == 0xFC) $n=5; # 1111110b
        else return false; # Does not match any model
        for ($j=0; $j<$n; $j++) { # n bytes matching 10bbbbbb follow ?
            if ((++$i == $length) || ((ord($str[$i]) & 0xC0) != 0x80))
                return false;
    return true;

function is_utf8($str) {
    $c=0; $b=0;
    for($i=0; $i<$len; $i++){
        if($c > 128){
            if(($c >= 254)) return false;
            elseif($c >= 252) $bits=6;
            elseif($c >= 248) $bits=5;
            elseif($c >= 240) $bits=4;
            elseif($c >= 224) $bits=3;
            elseif($c >= 192) $bits=2;
            else return false;
            if(($i+$bits) > $len) return false;
            while($bits > 1){
                if($b < 128 || $b > 191) return false;
    return true;

If anyone is interested I found a great example page to use when testing UTf-8.



Do I need to convert everything that I receive from the user agent (HTML form's & URI) to UTF-8 when the page loads

No. The user agent should be submitting data in UTF-8 format; if not you are losing the benefit of Unicode.

The way to ensure a user-agent submits in UTF-8 format is to serve the page containing the form it's submitting in UTF-8 encoding. Use the Content-Type header (and meta http-equiv too if you intend the form to be saved and work standalone).

I have heard that you should mark you forms as UTF-8 also (accept-charset="UTF-8")

Don't. It was a nice idea in the HTML standard, but IE never got it right. It was supposed to state an exclusive list of allowable charsets, but IE treats it as a list of additional charsets to try, on a per-field basis. So if you have an ISO-8859-1 page and an “accept-charset="UTF-8"” form, IE will first try to encode a field as ISO-8859-1, and if there's a non-8859-1 character in there, then it'll resort to UTF-8.

But since IE does not tell you whether it has used ISO-8859-1 or UTF-8, that's of absolutely no use to you. You would have to guess, for each field separately, which encoding was in use! Not useful. Omit the attribute and serve your pages as UTF-8; that's the best you can do at the moment.

If a UTF string is improperly encoded will something go wrong

If you let such a sequence get through to the browser you could be in trouble. There are ‘overlong sequences’ which encode an low-numbered codepoint in a longer sequence of bytes than is necessary. This means if you are filtering ‘<’ by looking for that ASCII character in a sequence of bytes, you could miss one, and let a script element into what you thought was safe text.

Overlong sequences were banned back in the early days of Unicode, but it took Microsoft a very long time to get their shit together: IE would interpret the byte sequence ‘xC0xBC’ as a ‘<’ up until IE6 Service Pack 1. Opera also got it wrong up to (about, I think) version 7. Luckily these older browsers are dying out, but it's still worth filtering overlong sequences in case those browsers are still about now (or new idiot browsers make the same mistake in future). You can do this, and fix other bad sequences, with a regex that allows only proper UTF-8 through, such as this one from W3.

If you are using mb_ functions in PHP, you might be insulated from these issues. I can't say for sure as mb_* was unusable fragile when I was still writing PHP.

In any case, this is also a good time to remove control characters, which are a large and generally unappreciated source of bugs. I would remove chars 9 and 13 from submitted string in addition to the others the W3 regex takes out; it is also worth removing plain newlines for strings you know aren't supposed to be multiline textboxes.

Was UTF-16 written to address a limit in UTF-8?

No, UTF-16 is a two-byte-per-codepoint encoding that's used to make indexing Unicode strings easier in-memory (from the days when all of Unicode would fit in two bytes; systems like Windows and Java still do it that way). Unlike UTF-8 it is not compatible with ASCII, and is of little-to-no use on the Web. But you occasionally meet it in saved files, usually ones saved by Windows users who have been misled by Windows's description of UTF-16LE as “Unicode” in Save-As menus.


This is very inefficient compared to the regex!

Also, make sure to use utf8_unicode_ci on all of your tables.

You can actually sort of get away without this, treating MySQL as a store for nothing but bytes and only interpreting them as UTF-8 in your script. The advantage of using utf8_unicode_ci is that it will collate (sort and do case-insensitive compares) with knowledge about non-ASCII characters, so eg. ‘?’ and ‘?’ are the same character. If you use a non-UTF8 collation you should stick to binary (case-sensitive) matching.

Whichever you choose, do it consistently: use the same character set for your tables as you do for your connection. What you want to avoid is a lossy character set conversion between your scripts and the database.

Thursday, August 4, 2022

As mentioned in the comments this is because there are usually at least two php.ini files: one for the command line version and one for the Apache plugin. You need to make sure you edit the right one.

This is not an uncommon problem and I've certainly been bitten by it before.

Wednesday, September 21, 2022
$utf8string = html_entity_decode(preg_replace("/U+([0-9A-F]{4})/", "&#x\1;", $string), ENT_NOQUOTES, 'UTF-8');

is probably the simplest solution.

Thursday, August 4, 2022

First question: it depends on what exactly goes in the string.

In PHP (up to PHP5, anyway), strings are just sequences of bytes. There is no implied or explicit character set associated with them; that's something the programmer must keep track of. So, if you only put valid UTF-8 bytes between the quotes (fairly easy if the file itself is encoded as UTF-8), then the string will be UTF-8, and you can safely use mb_strlen() on it.

Also, if you're using mbstring functions, you need to explicitly tell it what character set your string is, either with mbstring.internal_encoding or as the last argument to any mbstring function.

Second question: yes, with caveats.

Two strings that are both independently valid UTF-8 can be safely byte-wise concatenated (like with PHP's . operator) and still be valid UTF-8. However, you can never be sure, without doing some work yourself, that a POSTed string is valid UTF-8. Database strings are a little easier, if you carefully set the connection character set, because most DBMSs will do any conversion for you.

Monday, November 7, 2022

Got it!!! Looks like this is a problem when updating MySQL versions, in my case, from 3.2 to 4.1.22. The thing is that the privilege tables must be updated too, but somehow they weren't. So, I logged into the server as root via SSH and executed the command:

mysql_fix_privilege_tables --password=[your_root_pass]

That's it! Hope it helps others in my situation...

Saturday, October 22, 2022
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