fwrite（）执行两次写文本

Question

<?php

/**
 * Debug helper
 */
class Debug
{
    /**
     * Debug initiated?
     *
     * @var boolean
     */
    static private $_initiated = false;

    /**
     * Initialize debugging
     */
    static public function main()
    {
        /* Init once */
        if (self::$_initiated)
        {
            return;
        }
        self::$_initiated = true;

        /* Setup error/exception handling */
        $handler = array(__CLASS__, IN_DEV ? 'devhdl' : 'prodhdl');
        set_error_handler($handler);
        set_exception_handler($handler);
    }

    /**
     * Development mode handler
     */
    static public function devhdl()
    {
        /* Init */
        $msg = self::_parseError(func_get_args());

        /* Build output */
        $out = sprintf('<pre>%s</pre>', print_r($msg, 1));

        /* Send */
        die($out);
    }

    /**
     * Production mode handler
     */
    static public function prodHdl()
    {
        /* Init */
        $msg = self::_parseError(func_get_args());
        $sep = "\n" . str_repeat('-', 60) . "\n";

        /* Log message */
        $log = fopen(DEV_LOG, 'a+b');
        flock($log, LOCK_EX);
        fwrite($log, $msg . $sep);
        flock($log, LOCK_UN);
        fclose($log);

        /* Send */
        die('System error');
    }

    /**
     * Error parser
     *
     * @param array $params <p>Input arguments</p>
     * @return string <p>Error message</p>
     */
    static private function _parseError($params)
    {
        /* Invalid call? */
        if (!(is_array($params) && isset($params[0])))
        {
            die('System error');
        }

        /* Exception has been thrown? */
        if ($params[0] instanceof Exception)
        {
            return $params[0]->getMessage() . "\n\n" . $params[0]->getTraceAsString();
        }

        /* Process error */
        if (count($params) == 5)
        {
            if (!(error_reporting() & $params[0]))
            {
                return 'unhandlable_error';
            }

            return $params[1] . "\n\n" . $params[2] . ':' . $params[3];
        }

        return 'unknown_error';
    }
}

?>

当IN_DEV == 1 - 工作正常时

当IN_DEV == 0 - 工作正常时，但fwrite会写两次日志消息

WinXP / SP3，PHP / 5.3

我应该如何解决这个问题？