Interface example

ilogger.php

<?php
  // Logger interface.
  interface iLogger
  {
    public function __construct();
    public function write($str);
  }

logger_console.php

<?php
  // Example logger implementation that outputs to the console.
  class Logger_Console implements iLogger
  {
    public function __construct()
    {
      // Do nothing.
    }
    
    public function write($str)
    {
      echo $str.PHP_EOL;
    }
  }

logger_file.php

<?php
  // Example logger implementation that logs to a file.
  class Logger_File implements iLogger
  {
    protected $_fp = null;
    
    public function __construct()
    {
      $this->_fp = fopen('/var/log/useful.log', 'wt');
    }
    
    public function write($str)
    {
      fwrite($this->_fp, $str.PHP_EOL);
    }
  }

main.php

<?php
  // This script creates a user first with the console logger, then with the file logger.
  $user = new User(new Logger_Console());
  $user->create('Stuart', 'qwertyuiop');

  $user = new User(new Logger_File());
  $user->create('Tedd', 'zxcvbnm');

user.php

<?php
  // Example class that takes a logger implementation.
  class User
  {
    protected $_logger = null;
    
    public function __construct(iLogger $logger)
    {
      $this->_logger = $logger;
    }
    
    public function create($username, $password)
    {
      $_logger->write('Creating user named "'.$username.'" with the password "'.$password.'"');
    }
  }