Symfony __PHP_Incomplete_Class could not be converted to string - php

Question

When I submit my login form I get that error. I read here that this could be caused by trying to
deserialze object without loaded class for that object
however because the code is hidden away within symfony internals I can't work out why a class wouldn't be loaded in that instance. Profiler seems to say that no queries have been performed, yet if I put in the wrong credentials it says "invalid credentials" so it must be accessing the DB correctly.
User class
<?php
namespace App\Entity;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
use Symfony\Component\Security\Core\User\UserInterface;
/**
* #ORM\Entity(repositoryClass="App\Repository\UserRepository")
* #UniqueEntity("email", message="This email is already in use.")
* #UniqueEntity("username", message="This username is already in use")
*/
class User implements UserInterface, \Serializable
{
private $roles = "ROLE_USER";
/**
* #ORM\Column(name="salt",type="string", length=255)
*/
private $salt = "saltyboye";
/**
* #ORM\Id()
* #ORM\GeneratedValue()
* #ORM\Column(type="integer")
*/
private $id;
/**
* #ORM\Column(name="username",type="string", length=255, unique=true)
*/
private $username;
/**
* #ORM\Column(name = "password", type="string", length=255)
*/
private $password;
/**
* #ORM\Column(name="email", type="string", length=255, unique=true)
*/
private $email;
/**
* #ORM\Column(type="datetime")
*/
private $registeredOn;
/**
* #ORM\Column(type="integer", nullable=true)
*/
private $referrer;
/**
* #ORM\Column(type="smallint")
*/
private $entries;
/**
* #ORM\Column(type="string", length=3)
*/
private $currency;
/** #see \Serializable::serialize() */
public function serialize()
{
return serialize(array(
$this->registeredOn,
$this->id,
$this->email,
$this->username,
$this->password,
$this->roles,
$this->referrer,
$this->currency,
$this->entries,
$this->salt));
}
public function unserialize($serialized)
{
list (
$this->id,
$this->email,
$this->username,
$this->password,
$this->roles,
$this->referrer,
$this->currency,
$this->entries,
$this->salt) = unserialize($serialized, array('allowed_classes' => false));
}
public function eraseCredentials()
{
}
public function getRoles()
{
return array("ROLE_USER");
}
public function getSalt()
{
return $this->salt;
}
public function getId()
{
return $this->id;
}
public function getUsername(): ?string
{
return $this->username;
}
public function setUsername(string $username): self
{
$this->username = $username;
return $this;
}
public function getPassword(): ?string
{
return $this->password;
}
public function setPassword(string $password): self
{
$this->password = $password;
return $this;
}
public function getEmail(): ?string
{
return $this->email;
}
public function setEmail(string $email): self
{
$this->email = $email;
return $this;
}
public function getRegisteredOn(): ?\DateTimeInterface
{
return $this->registeredOn;
}
public function setRegisteredOn(\DateTimeInterface $registeredOn): self
{
$this->registeredOn = $registeredOn;
return $this;
}
public function getReferrer(): ?int
{
return $this->referrer;
}
public function setReferrer(?int $referrer): self
{
$this->referrer = $referrer;
return $this;
}
public function getEntries(): ?smallint
{
return $this->entries;
}
public function setEntries($entries): self
{
$this->entries = $entries;
return $this;
}
public function setCurrency($currency): self
{
$this->currency = $currency;
return $this;
}
public function getCurrency(): ?string
{
return $this->currency;
}
}
Security.yaml
security:
hide_user_not_found: false
# https://symfony.com/doc/current/security.html#where-do-users-come-from-user-providers
encoders:
App\Entity\User: sha256
providers:
in_memory: { memory: ~ }
main_db_provider:
entity:
class: App\Entity\User
property: username
firewalls:
dev:
pattern: ^/(_(profiler|wdt)|css|images|js)/
security: false
main:
# anonymous: true
pattern: ^/ #test
anonymous: ~
form_login:
login_path: login
check_path: login
csrf_token_generator: security.csrf.token_manager
provider: main_db_provider
# activate different ways to authenticate
# http_basic: true
# https://symfony.com/doc/current/security.html#a-configuring-how-your-users-will-authenticate
# form_login: true
# https://symfony.com/doc/current/security/form_login_setup.html
# Easy way to control access for large sections of your site
# Note: Only the *first* access control that matches will be used
access_control:
- { path: ^/$, roles: ROLE_USER }
# - { path: ^/$, roles: ROLE_USER }

Answer 1

The serialize function doesnot match the unserialize function! You need to change it:
/** #see \Serializable::serialize() */
public function serialize()
{
return serialize(array(
$this->registeredOn,
$this->id,
$this->email,
$this->username,
$this->password,
$this->roles,
$this->referrer,
$this->currency,
$this->entries,
$this->salt));
}
public function unserialize($serialized)
{
list (
$this->registeredOn, # <--- look here it is missing
$this->id,
$this->email,
$this->username,
$this->password,
$this->roles,
$this->referrer,
$this->currency,
$this->entries,
$this->salt) = unserialize($serialized, array('allowed_classes' => false));
}