单例模式

<?php
 
class Singleton
{
    private static $instance = null;
 
    private function __construct()
    {
        // Private constructor so nobody else can instance it
    }
 
    public static function getInstance()
    {
        if (self::$instance == null) {
            self::$instance = new Singleton();
        }
 
        return self::$instance;
    }
 
    protected function __clone()
    {
        throw new \Exception('Cannot clone');
    }
 
    protected function __wakeup()
    {
        throw new \Exception('Cannot unserialize');
    }
}
 
$singleton1 = Singleton::getInstance();
$singleton2 = Singleton::getInstance();
 
var_dump($singleton1 === $singleton2); // bool(true)

<?php
 
class Singleton
{
    private static $instances = [];
 
    private function __construct()
    {
        // Private constructor so nobody else can instance it
    }
 
    public static function getInstance()
    {
        $subClass = static::class;
        if (!isset(self::$instances[$subClass])) {
            self::$instances[$subClass] = new static;
        }
 
        return self::$instances[$subClass];
    }
 
    protected function __clone()
    {
        throw new \Exception('Cannot clone');
    }
 
    protected function __wakeup()
    {
        throw new \Exception('Cannot unserialize');
    }
}
 
class SingletonChildA extends Singleton
{
}
 
class SingletonChildB extends Singleton
{
}
 
$singletonA1 = SingletonChildA::getInstance();
$singletonA2 = SingletonChildA::getInstance();
 
$singletonB1 = SingletonChildB::getInstance();
$singletonB2 = SingletonChildB::getInstance();
 
var_dump($singletonA1 === $singletonA2); // true
var_dump($singletonB1 === $singletonB2); // true
var_dump($singletonA1 === $singletonB1); // false
var_dump($singletonA2 === $singletonB2); // false