Laravel/Illuminate encryption length checksum?

Question

I use the following source to encrypt and decrypt a string in Laravel:

    $newEncrypter = new \Illuminate\Encryption\Encrypter( '098f6bcd4621d373cade4e832627b4f6', 'aes-256-cbc' );
    $encrypted = $newEncrypter->encrypt('testvalue');
    $decrypted = $newEncrypter->decrypt( $encrypted );
    //$decrypted value is testvalue -> everything is fine

If I try to decrypt this in my C# Application the decrypted value is: s:9:"testvalue";. I think this is an length checksum for the string value. Can anyone confirm this?