问解析错误:语法错误，意外的'$_POST‘(T_VARIABLE)错误，我就是不明白为什么？

EN

<?php
session_start();
require 'db_con.php';
class logg{

    private $db;
    private $username = $_POST['usr_name']; 
    private $password = $_POST['usr_pass'];
    private $query = "SELECT * FROM user WHERE nick='$username'";

function __construct(){
    $this->db = new DB();
}

function loggIn(){
    try{
        $res = $this->db->fetch($query)
        foreach($res as $row) {
            $dbusername = $row['nick'];
            $dbpassword = $row['pass']; }
        if ($username == $dbusername && $password == $dbpassword) {
            $_SESSION['username'] = $dbusername;
            header('Location: index.php');
        } catch (Exception $e){
        echo'Wrong login information!'
        }
    }
}   

function loggOut(){
    session_destroy();
}
}

?>

    private $username = $_POST['usr_name']; 
    private $password = $_POST['usr_pass'];

    ( ! ) Parse error: syntax error, unexpected '"' in