dependenyinjection.php

<?php

class ObjectWithDatabase
{
    private $_db;

    public function __construct( Database_Interface $database = null )
    {
        if ($database === null) {
            $this->_db = new Database_Mysql();
        } else {
            $this->_db = $database;
        }
    }
}

$database = new Database_Sqlite();
$obj      = new ObjectWithDatabase( $database );

// without params: default adapter
$obj      = new ObjectWithDatabase();

?>