singleton.php

<?php

class FooClass {

	public static function get_instance() {

		static $instance;

		if ( ! $instance ) {
			$variant = 'A';

			switch ( $variant ) {
				case 'A':
					$instance = ( new self() )->init();
					break;

				case 'B':
					$instance = new self();

					$instance->init();
					break;
			}
		}

		return $instance;
	}

	private function init() {

		$a = foo_func();

		return $this;
	}
}

function foo_func() {

	return FooClass::get_instance();
}

foo_func();