Singletons, with Dependancy Injection and unit test support!

class_abstract_writer.php

<?php

abstract class AbstractWriter implements Writer {

	protected $counter = 0;
	
	public function write( $file, $message ) {
		$fp = fopen( $file, 'a' );
		
		fwrite( $fp, $message . "\r\n" );
		
		fclose( $fp );
	}
	
	public function increment( $step ) {
		$this->counter += $step;
	}
	
	public function get_counter() {
		return $this->counter;
	}

}

class_writer.php

<?php

final class Singleton_Demo extends Abstract_Demo {
	use Singleton;
}

interface_writer.php

<?php

interface Writer {

	function write( $file, $message );
	
	// These methods will be used to demonstrate state
	function increment( $step );
	function get_counter();

}

trait_singleton.php

<?php

trait Singleton {
	
	protected static $instance;

	protected function __construct() {}
	
	/**
	*	Return class instance
	*	@return object
	**/
	public static function Instance() {
		if(!(static::$instance instanceof static)) {
			static::$instance = new static();
		}
		return static::$instance;
	}

}