Is the PHP function error_log() safe to use?

I want to insure that there are no race conditions introduced by using a custom PHP error handler. To that end I want to know if I can rely on error_log() or if I need to use some other file-locking method to insure errors are logged correctly.

How does the default PHP error handler work? Is it safe from race conditions?

For example, do I have to lock the file (which might cause errors to be lost in this simple version)

function log_error($message)
{
    if(! $fp = @fopen('/path/to/error.log', 'a'))
    {
        return FALSE;
    }

    flock($fp, LOCK_EX);
    fwrite($fp, $message);
    flock($fp, LOCK_UN);
    fclose($fp);

    return TRUE;
}

or can I just call error_log?

error_log($message, 0);