PHP 数组转json格式，key的保存问题

 <?php
 $arr = [
     2,
     3,
 ];
 echo print_r($arr,true);
 echo json_encode($arr);
 echo "\n\n";

 $arr = [
     0 => 2,
     1 => 3,
 ];
 echo print_r($arr,true);
 echo json_encode($arr);
 echo "\n\n";

 $arr = [
     0 => 2,
     2 => 3,
 ];
 echo print_r($arr,true);
 echo json_encode($arr);
 echo "\n\n";

 $arr = [
     1 => 2,
     2 => 3,
 ];
 echo print_r($arr,true);
 echo json_encode($arr);
 echo "\n\n";

 $arr = [
     '0' => 2,
     '1' => 1,
 ];
 echo print_r($arr,true);
 echo json_encode($arr);
 echo "\n\n";

 $arr = [
     '1' => 2,
     '2' => 1,
 ];
 echo print_r($arr,true);
 echo json_encode($arr);
 echo "\n\n";

 $arr = [
     '0' => 2,
     '2' => 1,
 ];
 echo print_r($arr,true);
 echo json_encode($arr);
 echo "\n\n";