Singletone trait

class-demo.php

<?php
/**
 * The Demo class.
 *
 * @package xyz
 */

namespace UtkarshPatel\Demo

/**
 * Class Demo
 */
class Demo {

	use UtkarshPatel\Traits\Singleton;

	/**
	 * Initialization.
	 */
	protected function init() {
		/*
		 * Add everything that you usually add to constructor
		 * In case of inheritance parent class should have this trait and not child else it not work since constructor is not calling parent.
		 */
		add_filter( 'init', array( $this, 'my_method' ) );
	}

	public function my_method() {

	}

}

trait-singleton.php

<?php
/**
 * Trait Singleton
 *
 * @package XYZ
 */

namespace UtkarshPatel\Traits;

/**
 * Trait Singleton
 */
trait Singleton {

	/**
	 * Instance of class.
	 *
	 * @var $instance
	 */
	protected static $instance;

	/**
	 * Create instance only once.
	 *
	 * @return static Current class object.
	 */
	final public static function get_instance() {
		return isset( static::$instance )
			? static::$instance
			: static::$instance = new static();
	}

	/**
	 * Singleton constructor.
	 */
	final private function __construct() {
		$this->init();
	}

	/**
	 * Action / Filters to be declare here.
	 */
	protected function init() {
	}

	/**
	 * Make private magic method wakeup.
	 */
	final private function __wakeup() {
	}

	/**
	 * Make private magic method clone.
	 */
	final private function __clone() {
	}
}