You can test for them explicitly or rely on PHP to return a default empty value when you reference them.

您可以显式地对它们进行检测，或在您引用它们时让 PHP返回缺省的空值。

youdao

Return to the Default Messaging Provider dialog and select JMS activation specification => New.

返回到 Default Messaging Provider 对话框并选择 JMS activation specification => New。

youdao

To return the default value of true click Complete.

若要返回缺省值 true，请单击 Complete。

youdao

Without necessary reforms, eventually this vicious circle of stagdeflation would force Italy to exit EMU, return to the Lira and default on its Euro debts.

FORBES: Friedman and Roubini on the Euro Breakup

Gold could return to rallying if more European countries than Greece default, as investors would again view gold as a safe haven.

FORBES: FOCUS: European Situation Overshadowing Economic News, Markets

In consequence pension funds and banks will no longer be able to give up some of their bond yield in return for insuring the capital against government default.

FORBES: ECB Cuts Interest Rates As G20 Meets In Cannes