how to handle json encoded php mysql array

Try using console.log(returnData) in Chrome or Firefox (or any decent browser) and have a look at the developer tools / console output to get a glimpse at your data in a structured way.

document.write() cannot provide readable structured output from a javascript object. Therefore it is returning [object Object] as a representation of what is truly there.

What happened in your case is that the encapsulated json_encode() calls basically do this: the second call (inside the first) will already return a valid string of json data and the first call will then wrap entire json object within another one, such as this:

// begin json
{
    // begin encapsulated json
    {
        header: 'any data'
    }
    // end of encapsulated json
}    
// end json

This is not what you want. Iterating over data returned like this on the first level will give you only one iteration: the entire json object wrapped within the outer one. Therefore .header cannot return anything, because it is simply not there.

There is no need to wrap a second json_encode() within a first, because json_encode() will already recursively convert the provided data into a JSON object definition string, no matter how many levels it has (or maybe there is some limit, i don't know, probably based on the architecture of the system the client is running on).

Therefore:

1. Remove the outer json_encode() and keep only one call to this function (as you had in the beginning)
2. Use console.log() to debug your data and see what is being returned on which level
3. Adapt your code accordingly, so that you end up being able to access the values by the correct keys

Unless you provide more information (an example of your JSON data for instance) it is hard to help you with step number 3.

it is returning the array of objects, I believe that if you use the JSON library solve your case

In your php file,

echo json_encode($result);

And in jquery, in the success callback of $.ajax,

returnData.forEach(function(elem) {
    console.log(elem);
    // do something with the individual objects
});

You can directly check what is the response using,

console.log(returnData);

Don't do double json_encode! In javascript, you recieving object, not string, so you can't simply write it. If you need string representation of object in html, use JSON.stringify(data). But you propably need to process object in another way.

Dealing with JSON data

  $.getJSON( "ajax/test.json", function( data ) {
        var items = [];
        $.each( data, function( key, val ) {
        items.push( "<li id='" + key + "'>" + val + "</li>" );
    });

This example has been taken from this page. I know the function it's a bit different but the concepts are the same.

Assuming that your JSON is valid and everything else as well, your returning data is an object. So, you'll have to deal with iteration to get some values. In that example, you can see the $.each loop that allows you to get what you need.

{
   "one": "Singular sensation",
   "two": "Beady little eyes",
   "three": "Little birds pitch by my doorstep"
}

This is a JSON example. With the previous example, key will be one, two and three while val will be "Singular sensation", "Beady little eyes" or "Little birds pitch by my doorstep". Now you know how to access your data.

Since I don't know your JSON structure, I can't help you any further. If you update your question with some JSON code and what you're trying to do, I'll be more specific.

try this :

PHP:

header('Content-Type','application/json');
echo json_encode($result,JSON_FORCE_OBJECT);

ajax:

$.ajax({
    url : "helpers/data.php",
    dataType : "json"
}).done(function(data) {
    alert(data);
    return callback(null, data);
}).fail(function() {
    return callback("AJAX error");
});