($arr,"name","a");
$subTitle=data_get($arr,"title.sub","a");
$mainTitle=data_get($arr...null的情况下,默认值不起作用,返回null
$test2=data_get($arr,"test","a") ??...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,?...:进行配合 ,类似下面这样
desc=data_get(arr,"desc","a") ?: "a";