I'm not sure of implications yet, but this nonce appears to be predictable. If non-predictability is important (as it is for most nonces) suggest change to openssl_random_pseudo_bytes.
The server_nonce field is only used inside the synclib code to keep track of entries in the queue table.
I think it's ok to have it predictable here, though it might still be worth to change to something better. The problem with openssl_random_pseudo_bytes() is that it would make us non-compatible with php 5.2 (though that might not be an issue any more?)
PHP 5.2 reached EOL over three years ago: http://php.net/eol.php