There might be some error codes missing from the package depending on the MQ client version you are using. To prevent further versions from returning an empty error message simply use a function such as:

<?php
    //... wrapper class ...

    public function getErrorMessage ($errorCode) {
        $errorString = mqseries_strerror($errorCode);
        if (!empty($errorString)) {
            return $errorString;
        } else {
            switch ($errorCode) {
                // insert missing codes here
                case 2537:
                    return "Channel is not available.";
                case 2538:
                    return "Host is not available.";
                case 2539:
                    return "Channel configuration error.";
                case 2540:
                    return "Channel name is unknown.";
                default:
                    return "Unexpected error ({$errorCode})";
            }
        }
    }

    // ... wrapper class ...
?>