解析错误：语法错误，意外的'$ _POST'（T_VARIABLE）错误，我只是不明白为什么？

I am trying to write this simple class in php as follows:

<?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();
}
}

?>

but i keep getting the following error: Parse error: syntax error, unexpected '$_POST' (T_VARIABLE) and i just cant figure out why! Even if i remove theese rows:

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

i instead get the following error:

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

im certain the error is a real simple one, but I just can't seem to figure it out..! Any help appriciated!

Aside from a syntax error on your echo line, the actual problem resulting in this particular error is that you can only initialise class members like that with constants.

Since constants never begin with a $, your initialisations exhibit a syntax error.

You'll have to assign to those members in the class's constructor, instead, just like you do for $db.

You're missing a semi-colon here:

echo'Wrong login information!' // <-- HERE