The property you are trying to access is not defined and will cause unexpected behavior when used.

This issue will be raised in the following cases:

An undefined property is accessed.
A private property from a parent class is accessed.
A protected property of an instance of a class is accessed.
The variable whose property is being accessed is not an instance of a class, or is not defined.
Using properties via __get() or __set() magic methods.

Bad practice

class Admin
{
    public function getName(): string
    {
        return $this->name; // $name is not declared as a property of the class Admin
    }
}

class User
{
    private string $name = 'John Doe';
}

class Admin extends User
{
    public function getName(): string
    {
        return $this->name; // $name is a private property of the parent class
    }
}

class User
{
    protected string $name = 'John Doe';
}

class Admin
{
    public function getName(): string
    {
        $user = new User;

        return $user->name; // $name is a protected property defined in the User class, and cannot be accessed outside of it or its descendent classes
    }
}

class Admin
{
    public string $user = 'John';

    public function getName(): string
    {
        return $this->user->name; // $this->user is not an instance of any class
    }
}

Accesing magic properties:

Though in this example, the __set() method is implemented, it doesn't guarantee that properties(username in this case) being accessed will always be there!

class Admin
{
    public function __set(string $name, mixed $value): void
    {
        $this->{$name} = $value;
    }

    public function getUsername(): string
    {
        // invalid: property "username" doesn't exist
        return $this->username;
    }
}

$admin = new Admin();
// Pay attention to this line. If this line weren't present, the "getUsername" method wouldn't be able to find the "username" property.
$admin->username = 'john';
$admin->getUsername();

Recommended

class Admin
{
    public string $name = 'John Doe';

    public function getName(): string
    {
        return $this->name;
    }
}

class User
{
    protected string $name = 'John Doe';
}

class Admin extends User
{
    public function getName(): string
    {
        return $this->name;
    }
}

class User
{
    public string $name = 'John Doe';
}

class Admin
{
    public function getName(): string
    {
        $user = new User;

        return $user->name;
    }
}

class User
{
    public string $name = 'John Doe';
}

class Admin
{
    public ?User $user;

    public function __construct()
    {
        $this->user = new User;
    }

    public function getName(): string
    {
        return $this->user->name;
    }
}

Accesing magic properties:

Use @property PHPDoc tag at the class level to let analyzer know what properties it can access. It'll fix the issue causing this error.

/**
 * @property string $username
 */
class Admin
{
    public function __set(string $name, mixed $value): void
    {
        $this->{$name} = $value;
    }

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

References

rafandoo / HelpDeskRplus