Pseudo-singleton for WordPress.

gistfile1.php

<?php

class Example {
    public function __construct() {
        $this->instantiate();
    }   
    
    public function instantiate() {
        static $instance = null;
        
        if ( null === $instance ) {
            add_filter( 'example_instance', array( $this, 'instantiate' ) );
            $instance = $this;
        }
        
        return $instance;
    }
    
    public function do_something_useful() {
        
    }
    
    protected function do_something_in_secret() {
        
    }
}