How can I detect a malformed UTF-8 string in PHP?

Question

The iconv function sometimes gives me an error:

Notice:
iconv() [function.iconv]:
Detected an incomplete multibyte character in input string in [...]

Is there a way to detect that there are illegal characters in a UTF-8 string before sending data to inconv()?

Solution

First, note that it is not possible to detect whether text belongs to a specific undesired encoding. You can only check whether a string is valid in a given encoding.

You can make use of the UTF-8 validity check that is available in preg_match ^{[PHP Manual]} since PHP 4.3.5. It will return 0 (with no additional information) if an invalid string is given:

$isUTF8 = preg_match('//u', $string);

Another possibility is mb_check_encoding ^{[PHP Manual]}:

$validUTF8 = mb_check_encoding($string, 'UTF-8');

Another function you can use is mb_detect_encoding ^{[PHP Manual]}:

$validUTF8 = ! (false === mb_detect_encoding($string, 'UTF-8', true));

It's important to set the strict parameter to true.

Additionally, iconv ^{[PHP Manual]} allows you to change/drop invalid sequences on the fly. (However, if iconv encounters such a sequence, it generates a notification; this behavior cannot be changed.)

echo 'TRANSLIT : ', iconv("UTF-8", "ISO-8859-1//TRANSLIT", $string), PHP_EOL;
echo 'IGNORE   : ', iconv("UTF-8", "ISO-8859-1//IGNORE", $string), PHP_EOL;

You can use @ and check the length of the return string:

strlen($string) === strlen(@iconv('UTF-8', 'UTF-8//IGNORE', $string));

Check the examples on the iconv manual page as well.