PHP配列の簡単な配置

$arr = array(1=>"a","b","c","d");  echo $arr[3]; //打印c

 function boldTitle($inputText) {

         echo "<B>{$inputText}</B>";
       }

        date_default_timezone_set("UTC");
         
         $i = 0;

       for ($currentDate = date("U");date("l",$currentDate) != "Monday";$currentDate += (60*60*24)) {
          
          if ($i++%2==0) {

            echo "<li>".boldTitle(date("l",$currentDate))."</li>";

          }else {

              echo "<li>".date("l",$currentDate)."</li>";
          } 
        }

  $cities = array(

    "华北地区"=>array("安徽","湖北","河南"),
    "西北地区"=>array("甘肃","兰州")
);

   echo "华北地区:".$cities['华北地区'][0];