interface-example.php

<?php
interface LoggerInterface {
    public function __construct($prefix = null);
    public function log($message);
}

class Log1 implements LoggerInterface {

    protected $prefix;

    public function __construct($prefix = __CLASS__) {
        $this->prefix = $prefix;
    }

    public function log($message) {
        print $this->prefix . ' : ' . $message . "\n";
    }
}

class Log2 implements LoggerInterface {

    protected $prefix;

    public function __construct($prefix = __CLASS__) {
        $this->prefix = $prefix;
    }

    public function writeLog($message) {
        print 'not ' . $this->prefix . ' : ' . $message . "\n";
    }

    public function log($message) {
        return $this->writeLog($message);
    }
}

class Useful {
    protected $logger;

    public function __construct(LoggerInterface $logger = null) {
        if (empty($logger)) {
            $logger = new Log1('useful');
        }
        $this->logger = $logger;
    }

    public function make($something) {
        $this->logger->log($something);
    }
}


/////////

$useful = new Useful();
$useful->make('money');

$logger2 = new Log2('foobar');
$logger2->writeLog('now for something completely different');

$notuseful = new Useful('something');
$notuseful->make('loss');