Try:

$.ajax({
    url: "<?php echo site_url('demo/getPhotos/'); ?>",
    type: 'POST',
    data: form_data,
    dataType:"json",
    success: function(data) {
        alert(data[0]);
   }

On the PHP side, you'll be wanting to print:

print json_encode($photos);

Another thing you might try in order to better encapsulate your code, and as an example of further JSON gooodness, would be:

print json_encode(array("photolist"=>$photos,"photo_owner"=>"Me!"));

Then on the server, you'd access these with:

data.photolist[0]; //First photo
data.photo_owner;  //The owner of the photo set

data: { activitiesArray: activities },

That's it! Now you can access it in PHP:

<?php $myArray = $_REQUEST['activitiesArray']; ?>

Your array will always return undfined as the AJAX call is async, your function returns jsArray before it is set. and You don't need JSON.parse() as you have defined dataType as json in your ajax call. Pass a function to your getArr() function and use your data in that function.

function getArr(store_int, outputFn){ // what do you use store_int for?
   $.ajax({
       url: "fromSQL_store.php",
       data: {
           num: $("#store_int").val()
       },
       type: "GET",
       dataType: "json",
       success: function(data) {
           outputFn(data)
       },error: function(data){
           alert("123");
       }
   });
}

Then use it like

getArr('5', function (data) {
       console.log(data)
})

Console output

Defining $postData like :

<?php $postData = json_encode(array('FirstName'=>$_POST['user_name'])); ?>

You can send directly the Json without datatype like :

$.ajax({
    url: "http://XXXXXXXXX/api/CallBackForm",
    type: "POST",
    crossDomain: true,
    data: '<?php echo $postData; ?>',
});

Or if you have to use the dataType: 'json' and ContentType option (important for server side), you can parse the json before sending ajax like :

$.ajax({
    url: "http://XXXXXXXXX/api/CallBackForm",
    type: "POST",
    crossDomain: true,
    data: JSON.parse('<?php echo $postData; ?>'),
    contentType: 'application/json; charset=utf-8',
    dataType: 'json',
});

<?php // test.php
$myArray = array(1, 2, 3);
echo json_encode($myArray);
?>

HTML File:

$(function() {
    $.getJSON('http://localhost/test.php', function(data) {
        $(data).each(function(key, value) {
            // Will alert 1, 2 and 3
            alert(value);
        });
    });
});

use this

echo json_encode($array);

on server side

and

var arr=JSON.parse(result);

on client side

You could use JSON.stringify(array) to encode your array in JavaScript, and then use $array=json_decode($_POST['jsondata']); in your PHP script to retrieve it.

As Sergiu Costas pointer out, the best way is to JSON encode it and then decode, but you can also try:

$html="<center><a href=\"#\" id=\"check_spam\" onclick=\"$.ajax({type: 'POST', url: 'mcheck.php', data: '" . http_build_query(array('users' => $users)) . "', success: function(data){ $('#results').html(data);}});\">Check users for spam</a></center>";

If your $users array is:

$users = array( array('email' => 'a@email.com'), array('email' => 'b@email.com')));

then http_build_query('users' => $users) will generate:

users%5B0%5D%5Bemail%5D=a%40email.com&users%5B1%5D%5Bemail%5D=b%40email.com

which is the url-encoded version of:

 users[0][email]=a@email.com&users[1][email]=b@email.com

which will be decoded back to array in mcheck.php

Try below code

   <?php
        $var=5; 
        $myArray = array();
        while($var<10){
        $myArray[]=$var;
        $var++; 
        }
       $dataarray=array("myarray"=>$myArray);
        echo json_encode($dataarray);
        ?>

Jquery

  jQuery(document).ready(function(){
    jQuery("#previous").click(function(){
    var res = new Array(); 
        jQuery.getJSON("phparray.php", function(data) {
    var i= 0;
    while(i<data.myarray.length){
                res[i]=data.myarray[i];
             i++;
    }
  jQuery("#result").html(res[0]);
            });
        });

    });

Use JSON.

In the following example $php_variable can be any PHP variable.

<script type="text/javascript">
    var obj = <?php echo json_encode($php_variable); ?>;
</script>

In your code, you could use like the following:

drawChart(600/50, <?php echo json_encode($day); ?>, ...)

In cases where you need to parse out an object from JSON-string (like in an AJAX request), the safe way is to use JSON.parse(..) like the below:

var s = "<JSON-String>";
var obj = JSON.parse(s);