PHP Codes

<?php

    require_once('config.php');
    if(isset($_GET['what'])){

        $what = $_GET['what'];

        switch ($what) {
            case 'displayData':
                $what = displayData();
                break;
            case 'insertData':
                $what = insertData();
                break;
            case 'deleteData':
                $what = deleteData();
                break;
            case 'getData':
                $what = getData();
                break;
            default:
                # code...
                break;
        }
    }


    function displayData(){

        global $mysqli;
        $query = "SELECT * FROM users";
        $rs = $mysqli->query($query);

        $data = array();
        while($row = $rs->fetch_assoc()){
            $data[] = $row;
        }

        echo json_encode($data);
        $rs->free();
    }

    function insertData(){

        global $mysqli;
        $data = json_decode(file_get_contents("php://input"));
        $username = $mysqli->real_escape_string($data->username);
        $password = $mysqli->real_escape_string($data->password);

        $query = "INSERT INTO tbl_user(username, password)VALUES('$username', '$password')";

        if($mysqli->query($query)){
            echo "<p>New Record Successfully Created</p>";
        }

    }

    function deleteData(){

        global $mysqli;
        $data = json_decode(file_get_contents("php://input"));
        $id = $mysqli->real_escape_string($data->id);

        $query = "DELETE FROM tbl_user WHERE id = '$id'";

        if($mysqli->query($query)){
            echo "<p>Record Deleted</p>";
        }
    }

    function getData(){

        global $mysqli;
        $data = json_decode(file_get_contents("php://input"));
        $id = $mysqli->real_escape_string($data->id);

        $query = "SELECT * FROM tbl_user WHERE id ='$id'";
        $rs = $mysqli->query($query);
        $data = array();

        while($row = $rs->fetch_assoc()){
            $data[] = $row;
        }

        echo json_encode($data);
        $rs->free();
    }

    // function updateData(){

    //  global $mysqli;
    //  $data = file_get_contents("php://input");

    //  echo $id = $mysqli->real_escape_string($data)

    // }

?>