Encountering an unserialize(): Error at offset 0 of X bytes exception occurs when a ThinkPHP 6 application attempts to read session data originally generated by a ThinkPHP 5 application. The expection is triggered within the session driver's deserialization logic. Two primary incompatibilities cause this failure: prefix formatting and data serialization methods.

Session Prefix Discrepancy

In ThinkPHP 5, session data stored in Redis includes the configured session prefix followed by a pipe character. For example: app_prefix:|a:2:{s:4:"name";s:4:"test";}

ThinkPHP 6 stores the same data without this prefix: a:2:{s:4:"name";s:4:"test";}

When the newer framework passes the prefixed string directly to PHP's unserialize() function, the prefix characters cause the offset error.

Serialization Format Mismatch

If one application configures sessions to use PHP's native serialization while the other uses JSON, passing a JSON string to unserialize() will similarly trigger the offset error.

Implementing Cross-Version Compatibility

To resolve these issues, modify the session driver's deserialization routine to strip legacy prefixes and detect JSON formats before attempting standard unserialization.

protected function decodeSessionData($rawData)
{
    if (is_numeric($rawData)) {
        return $rawData;
    }

    $decoderCallback = $this->options['serialize'][1] ?? 'unserialize';

    // Fallback for JSON encoded sessions
    $Result = _decode($rawData, true);
    if (_last_error() === JSON_ERROR_NONE) {
        return $Result;
    }

    // Strip the legacy ThinkPHP 5 prefix
    $legacyPrefix = env('SESSION.PREFIX') . '|';
    if (str_contains($rawData, $legacyPrefix)) {
        $rawData = str_replace($legacyPrefix, '', $rawData);
    }

    return call_user_func($decoderCallback, $rawData);
}