<?php
$ary = array(1, 2, null, array("a", null, "c"));
var_dump(null2Str($ary, "なし"));
 
// nullを指定文字列に置換する関数
function null2Str(
  $in_array,// null値を含む配列
  $in_str// null値を変換したい文字列
){
  $n = function($n_array) use($in_str){
    if(is_array($n_array)){
      return array_map($n, $n_array);  //…(1)
    } else {
      if ($n_array === null){
        $n_array = $in_str;
      }
      return $n_array;
    }
  };
  return $n($in_array);
}

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