如何使用空值传递或回显数组的键

New to php. Please how do you select or output from an array, all the keys with NULL or empty values?

$emptyKeys = array_keys(array_filter($array, 'strlen'));

$emptyKeys contains an array of all the keys which contain empty values.

suppose that u have an array

<?php

    foreach($array AS $key=>$value){

        if(empty($value)){
          echo $key;
         }

    }
?>

or instead of echo you store your keys