[{"username":"User1","password":"Password"},
 {"username":"User5","password":"passWord"},]

<?php $username = $_POST["username"]; ?><br>
<?php $password = $_POST["password"]; ?><br>

<?php

    $currentData = file_get_contents('UserJSON.txt');
    $array_data = json_decode($currentData, true);
    $extra = array(
        'username' => $username,
        'password' => $password
    );
    $array_data[] = $extra;
    $final_data = json_encode($array_data);
    if(file_put_contents('UserJSON.txt',$final_data)) {
        print("working");
    }

?>

<?php
    $urlText = $_REQUEST["urlText"];
    $currentData = file_get_contents('UserJSON.txt');
    $array_data = json_decode($currentData, true);
    //for loop
    $array_data[i]['Posts'] = $urlText;
    //end for loop
    $final_data = json_encode($array_data);
    if(file_put_contents('UserJSON.txt',$final_data)) {
    }
?>

[{"username":"User1","password":"Password","Posts:{"This is a post"}},
{"username":"User5","password":"passWord"},"Posts:{"This is a post2"}}]