检查会话ID是否在此代码中

Question

How do I check if the session id is in this code? If it's not there how can I add it?

<?php
session_start();
if(isset($_REQUEST['login_button'])||$_REQUEST['auto']==1){
    require '../_database/database.php';
    $errmsg_arr = array();
    $errflag = false;
    $username=  mysqli_real_escape_string($database,$_REQUEST['username']);
    $password=  mysqli_real_escape_string($database,$_REQUEST['password']);
    if($username == '') {
        $errmsg_arr[] = 'Username missing';
        $errflag = true;
    }
    if($password == '') {
        $errmsg_arr[] = 'Password missing';
        $errflag = true;
    }
    if($errflag) {
        $_SESSION['ERRMSG_ARR'] = $errmsg_arr;
        session_write_close();
        header("location: authentication-check.php");
        exit();
    }
    $sql="SELECT user_username,user_password FROM user WHERE user_username='$username'AND user_password='$password'";
    $result=  mysqli_query($database,$sql) or die(mysqli_errno());
    $trws= mysqli_num_rows($result);
    if($trws==1){
        $rws=  mysqli_fetch_array($result);
        $_SESSION['user_username']=$rws['user_username'];
        $_SESSION['user_password']=$rws['user_password'];
        header("location:../home.php?user_username=$username&request=login&status=success");    
    }
    else {
        $errmsg_arr[] = 'user name and password not found';
        $errflag = true;
        if($errflag) {
            $_SESSION['ERRMSG_ARR'] = $errmsg_arr;
            session_write_close();
            header("location: ../components/authentication-check.php");
            exit();
        }
    }
}
?>

Answer 1

You can use Session ID like

session_start();    
echo session_id();

Or You can use $_SESSION['user_username'] as a unique identifier for session.