Ошибка в практическом примере "Замена ссылки значением"

<?php
class Customer {
    private $name;
    private $birthDate;

    public function getName() {
        return $this->name;
    }
    public function getBirthDate() {
        return $this->birthday;
    }
    public function setBirthDate(DateTime $birthDate) {
        $this->birthDate = $birthDate;
    }
    private function __construct($name) {
        $this->name = $name;
    }

    private static $instances = array();

    public static function get($name) {
        if (!isset($this->instances[$name])) {
            $value = new Customer($name);
            $this->instances[$name] = $value;
        }
        else {
            $value = $this->instances[$name]
    }
        return value;
    }
}

// Somewhere in client code
$john = Customer::get("John Smith");
$john->setBirthDate(new DateTime("1985-01-01"));

В статической функции get не может быть контекста $this, там должно быть self