Object functions don’t work inside sql_query in php

I want to create a query that Inserts user data to a database. The connection works fine but for some reason the functions from the object don’t return their values inside the sql_query and instead the value () is stored inside the columns of the database. The code used is the following:

function addAccountToDB($conn, $user) {
    //echo user->getUsername();
    $sql_query = "INSERT INTO users(username, email, address, password, telephone) VALUES ('$user->getUsername()', '$user->getEmail()', '$user->getAddress()', '$user->getPassword()','$user->getTelephone()')";
    if(mysqli_query($conn, $sql_query)) {
        //logIn($conn, $user, false);
        return true;
    }
    echo "query error " . mysqli_error($conn);
    return false;
}

And I get the following error:

Warning: Undefined property: User::$getUsername in D:Vs CodeComposting-Projectphpfunctions.php on line 45

Warning: Undefined property: User::$getEmail in D:Vs CodeComposting-Projectphpfunctions.php on line 45

Warning: Undefined property: User::$getAddress in D:Vs CodeComposting-Projectphpfunctions.php on line 45

Warning: Undefined property: User::$getPassword in D:Vs CodeComposting-Projectphpfunctions.php on line 45

Warning: Undefined property: User::$getTelephone in D:Vs CodeComposting-Projectphpfunctions.php on line 45

The echo right before the query works fine and the line 45 that the error is refering to is the $sql_query = "INSERT INTO users(username, email, address, password, telephone) VALUES ('$user->getUsername()', '$user->getEmail()', '$user->getAddress()', '$user->getPassword()','$user->getTelephone()')"; line.

The following code is the functions.php and classes.php files that are used:

functions.php:

<?php
session_start();
include("../php/connect.php");
include("../php/classes.php");

function addAccountToDB($conn, $user) {
    $sql_query = "INSERT INTO users(username, email, address, password, telephone) VALUES ('$user->getUsername()', '$user->getEmail()', '$user->getAddress()', '$user->getPassword()','$user->getTelephone()')";
    if(mysqli_query($conn, $sql_query)) {
        //logIn($conn, $user, false);
        return true;
    }
    echo "query error " . mysqli_error($conn);
    return false;
}
?>

classes.php;

<?php 
class User {
    private $username;
    private $email;
    private $address;
    private $password;
    private $telephone;

    public function __construct($username, $email, $address, $password, $telephone) {
        $this->username = $username;
        $this->email = $email;
        $this->address = $address;
        $this->password = $password;
        $this->telephone = $telephone;
    }

    public function getUsername() {
        return $this->username;
    }

    public function getEmail() {
        return $this->email;
    }

    public function getAddress() {
        return $this->address;
    }

    public function getPassword() {
        return $this->password;
    }

    public function getTelephone() {
        return $this->telephone;
    }
}
?>