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PHP unserialize bug and CodeIgniter

Using PHP’s serialize function to serialize and array or object where some element is a string with UTF8 data creates the serialized string properly. However, unserialize function fails to unpack that data. I ran into this when setting session flashdata from my CodeIgniter application. The solution I find to work (not sure if it’s perfect though) is to use mb_unserialize function (found in comments of PHP manual).

function mb_unserialize($serial_str) {
    $out = preg_replace('!s:(\d+):"(.*?)";!se', "'s:'.strlen('$2').':\"$2\";'", $serial_str );
    return @unserialize($out);

To use this with CodeIgniter’s session, just change Session’s _unserialize($data) function to use mb_unserialize instead of PHP’s original function. Grrr, I lost hours debugging this and finding a solution :(

Update: the problem became evident only because I used a wrong connection charset. Still I don’t like PHP returning an empty string when it cannot convert one character. I like the way some other programming languages are doing by placing a question mark (?) instead.

Milan Babuškov, 2010-07-02
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