Designed to be called as a singleton get_instance() or standard class constructor.

singletone_constructor.php

class singleton_constructor {
	public $foo = 'foo';
	public $bar = 'bar';
	public $foobar;

	private static $instance = null;

	private function __clone() {}
	private function __sleep() {}
	private function __wakeup() {}

	public static function get_instance() {
		if ( null === self::$instance ) {
			self::$instance = new self();
		}

		return self::$instance;
	}

	public function __construct( $initial_text ) {
		if ( null !== self::$instance ) {
			// Runs when there's a pre-existing instance.
			$this->foo    = self::$instance->foo;
			$this->bar    = self::$instance->bar;
			$this->foobar = self::$instance->foo . self::$instance->bar;
		} else {
			// Runs when it's a new instance.
			$this->foobar = $initial_text;
		}
		self::$instance = $this;

		// Run on every instance.
		$this->output( $this->foobar );
	}

	public function output( $text ) {
		echo $text;
	}
}