将javascript变量设置为PHP数组的特定索引

I have a one (very) big PHP array (with 649 indexes). I want to set a javascript variable to specific index of it and that index will depend on another variable. Is there any way? I know i can copy whole PHP array into JavaScript Array using json_encode($phpArray) , but i don't want whole array to be loaded on client side (for speed and some sort of security).

 $phpArray[0] = "i am first element";
 $phpArray[1] = "i am second element";
 /* ......... */
 $phpArray[100] = "i am 100th element";

and now, let us say, i want to get second element of $phpArray ? how can i do it?

Here is an example using jQuery $.post method:

$(document).ready(function(){
  var my_var;
  $.post("my-php-page.php",
         {my_index: 1},
         function(data) {
           my_var = data.my_value;
         },
         "json");
});

my-php-page.php

<?php
 $phpArray[0] = "i am first element";
 $phpArray[1] = "i am second element";
 /* ......... */
 $phpArray[100] = "i am 100th element";
 echo json_encode(array("my_value" => $phpArray[$_POST['my_index']]));
?>

Is this what you want?