如何在php中获取用户的用户ID?

im still developing android my android project i really need help, my problem is i couldn't get the user id of the user that login in my system so when they put a record a user id will attached to it data .. i want to do this to output their own data in my system. hope someone could help. its only php code thank you someone who would help.

<?php
    // Connection Details altered to hide actual values.
    $con = mysqli_connect("localhost", "db_user", "db_password", "db_name");

     $username = $_POST['username'];
    $password = $_POST['password'];

    $sql = "SELECT * FROM tbl_userinfo WHERE username = '$username' AND password='$password' LIMIT 1";
    $res = mysqli_query($con,$sql);


    $response = array();
    $response["success"] = false;  

    $row = mysqli_fetch_array($res);


     if(mysqli_num_rows($res)> 0){

        $response["success"] = true;
        session_start();
        $_SESSION['user_id'] =$userID;  
    }

    echo json_encode($response);

?>

thats for log in, here's for saving data..

    <?php
    session_start();
    $userID ="";
   // Connection Details altered to hide actual values.
    $con = mysqli_connect("localhost", "db_user", "db_password", "db_name");

   if(!isset($_SESSION['user_id'])){

    $userID = $_SESSION['user_id'];
    $checkdate = $_POST["checkdate"];
    $checkno = $_POST["checkno"];
    $datepaid = $_POST["datepaid"];
    $clientname = $_POST["clientname"];
    $bank = $_POST["bank"];
    $amount = $_POST["amount"];
    $status = "UNFINISHED";
    $statement = mysqli_prepare($con, "INSERT INTO tbl_checkinfo (user_id,checkno, checkdate, datepaid, clientname, bank, amount, status) VALUES (?,?, ?, ?, ?,?,?,?)");
    mysqli_stmt_bind_param($statement, "iissssis", $userID, $checkno, $checkdate, $datepaid, $clientname, $bank, $amount, $status);
    mysqli_stmt_execute($statement);

    $response = array();
    $response["success"] = false;  

    if($statement){
       $response["success"] = true;
    }



    echo json_encode($response);

}

?>

and for displaying user data.

<?php
     // Connection Details altered to hide actual values.
    $con = mysqli_connect("localhost", "db_user", "db_password", "db_name");

    $checkdate = $_POST["checkdate"];
    $checkno = $_POST["checkno"];
    $datepaid = $_POST["datepaid"];
    $clientname = $_POST["clientname"];
    $bank = $_POST["bank"];
    $amount = $_POST["amount"];
    $status = "UNFINISHED";
    $sql = "Select * from tbl_checkinfo";
    $result = mysqli_query($con, $sql);
   // $statement = mysqli_prepare($con, "Select * from tbl_checkinfo");
  //  mysqli_stmt_execute($statement);
   // mysqli_stmt_store_result($statement);
   // mysqli_stmt_bind_result($statement, $user_id, $checkdate, $checkno,     $datepaid, $clientname, $bank, $amount, $status);


    $response = array();
    $info=array();
    $flag = array();
    $response["success"] = false;  


if( mysqli_num_rows( $result ) > 0 ) {
    while($row = mysqli_fetch_array($result))
    {
        $flag[checkdate]=$row[checkdate];
        $flag[checkno]=$row[checkno];
        $flag[datepaid]=$row[datepaid];
        $flag[clientname]=$row[clientname];
        $flag[bank]=$row[bank];
        $flag[amount]=$row[amount];
        $flag[status]=$row[status];
        array_push($info, $flag);
    }
        $response["success"] = true;
        $response["message"] = $info;
        echo json_encode($response);
}
else
{
        $response["success"] = 0;
        $response["message"] = "No entries yet";
        echo json_encode($response);
}

?>

Firstly, when posting questions on public forums, please remove your host, DB name, password, etc from the code. :)

Secondly, try to print_r($row) and see on which index is the user id available, then in your code, add this line:

if(mysqli_num_rows($res)> 0){
        $response["success"] = true;
        $response["user_id"] = $row[USER_ID_INDEX];
        session_start();
        $_SESSION['user_id'] =$row[USER_ID_INDEX];  
    }

Where you defined $userID variable, You have to assign proper value to session variable,

if(mysqli_num_rows($res)> 0){

        $response["success"] = true;
        session_start();
        $_SESSION['user_id'] =$row[USER_ID_INDEX];  
    } 

$row['user_id_in_table'] should give you the id.