Just decode your json string and then use array push

$tempArray = json_decode($jsonstring, true);
array_push($tempArray, $your_data);

For your case

    $str = '{ 

"players":[
   {

        "name":"Moldova",
        "image":"/Images/Moldova.jpg",
        "roll_over_image":"tank.jpg"
   },
   {

        "name":"Georgia",
        "image":"/Images/georgia.gif",
        "roll_over_image":"tank.jpg"
   } ]}';


 $arr = json_decode($str, true);
 $arrne['name'] = "dsds";
 array_push( $arr['players'], $arrne );
 print_r($arr);

Just check value of print_r($arr); I hope this is what you want. :)

Simply, decode it using json_decode()

And append array to resulting array.

Again encode it using json_encode()

Complete code:

<?php
$arr = '[
   {
      "id":1,
      "name":"Charlie"
   },
   {
      "id":2,
      "name":"Brown"
   },
   {
      "id":3,
      "name":"Subitem",
      "children":[
         {
            "id":4,
            "name":"Alfa"
         },
         {
            "id":5,
            "name":"Bravo"
         }
      ]
   },
   {
      "id":8,
      "name":"James"
   }
]';
$arr = json_decode($arr, TRUE);
$arr[] = ['id' => '9999', 'name' => 'Name'];
$json = json_encode($arr);

echo '<pre>';
print_r($json);
echo '</pre>';

$data = json_decode($file, true);
//When TRUE, returned objects will be converted into associative arrays.
unset($file);

$newImage = array('image' => $link, 'folder' => $category);
//$newImage['image'] = $link;
//$newImage['folder'] = $category;
$result = array_merge($data, $newImage);
//don't need to cast

Reference PHP json_decode Manual

Edit The following code works (tested)

function addToJsonImageList($link, $category) {
    $file = file_get_contents('./images_list.json', true);
    $data = json_decode($file,true);
    unset($file);
    //you need to add new data as next index of data.
    $data[] = array('image' => $link, 'folder' => $category);
    $result=json_encode($data);
    file_put_contents('./images_list.json', $result);
    unset($result);
}

Edit 2 added lot of error reporting and debugging. Please let me know the output of the following. The code below is not tested (just typed in here).please fix if you find any syntax error. it is late here and can only check tomorrow my time, but can reply.

<?php
//display all errors and warnings
error_reporting(-1);
addToJsonImageList('test link', 'test category');

function addToJsonImageList($link, $category) {
    $file = file_get_contents('./images_list.json', true);
    if ($file===false)die('unable to read file');
    $data = json_decode($file,true);
    if ($data ===NULL)die('Unable to decode');
    unset($file);
    echo "data before\n";
    var_dump ($data);
    //you need to add new data as next index of data.
    $data[] = array('image' => $link, 'folder' => $category);
    echo "data after\n";
    var_dump ($data);
    $result=json_encode($data);
    if (file_put_contents('./images_list.json', $result) === false){
        die('unable to write file');
    }
    unset($result);
}
?>

Easy peasy lemon squeezy: http://www.php.net/manual/en/function.json-encode.php

<?php
$arr = array('a' => 1, 'b' => 2, 'c' => 3, 'd' => 4, 'e' => 5);

echo json_encode($arr);
?>

There's a post by andyrusterholz at g-m-a-i-l dot c-o-m on the aforementioned page that can also handle complex nested arrays (if that's your thing).

You are creating a new array and then appending it to the end of the existing array.

You just want to add a new key to the existing array:

$array['Gender'] = $gender;

You want to json_encode($json, JSON_FORCE_OBJECT).

The JSON_FORCE_OBJECT flag, as the name implies, forces the json output to be an object, even when it otherwise would normally be represented as an array.

You can also eliminate the use of array_push for some slightly cleaner code:

$json[] = ['ip' => $ip, 'port' => $port];

You're treating this like string manipulation, which is the dead wrong way to go about it. You need to combine the two structures while they're objects, before you re-encode them to JSON.

These three lines...

$data = json_encode($data);
$oldData = json_encode($oldData);
file_put_contents('base.json', '['.$data.','.$oldData.']');

Should be rewritten as...

// Create a new array with the new data, and the first element from the old data
$newData = array($data, $oldData[0]);
$newData = json_encode($newData);
file_put_contents('base.json', $newData);

Add $var[] = array("total"=>"3000"); just before you encode to JSON

$var=array("code"=>"100");
$sql = SELECT order as orderID, pub as orderCode from cart
while ($row = mysql_fetch_assoc ($sql) {
  $row['total'] = '3000' ;
  $var[] = $row;
}
echo '{"status":'. json_encode($var).'}';

The idea is to need to convert the JSON to array using json_decode, manipulate the fields, and encode it back in JSON format.

Try this:

$decoded_json = json_decode($json1, true);
$decoded_json[0]['fields'] = json_decode($json2, true);
$finalJSON = json_encode($decoded_json, JSON_PRETTY_PRINT);