,"title.main","a");//如果没有这个key的情况下,默认值起作用,返回string(1) "a"
$test=data_get($arr,"test","a");//如果是...null的情况下,默认值不起作用,返回null
$test2=data_get($arr,"test","a") ??..."a";//如果是null的情况下,??可以起作用,返回string(1) "a"
$desc=data_get($arr,"desc","a") ?...: "a";//如果是""的情况下,?:可以起作用,返回string(1) "a"
$num=data_get($arr,"num","a") ?: 100;//如果是0的情况下,?...data_get 与 ?