A Better WordPress Singleton

improved-boot.php

call_user_func(array(new PluginClass(__FILE__), 'boot'));

improved-singleton.php

class PluginClass
{
    protected static $instance = null;
    
    public function __construct($file)
    {
        if (static::$instance !== null) {
            throw new Exception;
        }

        static::$instance = $this;
    }
    public static function get()
    {
        return static::$instance;
    }
}

normal-singleton-and-boot.php

class PluginClass
{
    public static $instance = null;
    
    public static function init()
    {
        if ( null === self::$instance ) {
        self::$instance = new PluginClass();
        self::$instance->boot();
    }
        return self::$instance;
    }
    
    protected function __construct()
    {
        // Startup
    }
    
    protected function boot()
    {
        // Boot
    }
}

PluginClass::init();

I've been using it as a trait, and checking for constraints at the same time.

https://wp-helpers.com/2021/03/23/using-the-singleton-pattern-to-deal-with-constraints/