gistfile1.php

<?php

// Some logger interface

interface LoggerInterface
{
    public function log($message);
}

// Some class that relies upon the LoggerInterface interface

class Foo
{
    protected $logger;
    public function __construct(LoggerInterface $logger)
    {
        $this->logger = $logger;
    }
    
    public function bar()
    {
        // Do domething, then write a log.
        // $this->logger is guaranteed to have a log() method because it
        // implements the LoggerInterface
        $this->logger->log('Doing something');
    }
}

// Some classes that implement the LoggerInterfaces
class FSLogger implements LoggerInterface
{
    public function log($message)
    {
        // some code to write a log entry to the filesystem
    }
}

class DBLogger implements LoggerInterface
{
    public function log($message)
    {
        // some code to write a log entry to a database
    }
}

// Foo can use either LoggerInterface implementation
$foo = new Foo(new DBLogger);
$foo->bar();

// or
$foo = new Foo(new FSLogger);
$foo->bar();

// And then of course using DI and IoC containers you would have
// these dependencies configurable somewhere and then simply call
// something like:
$foo = DI::get('foo');
$foo->bar();