PHP Abstract Class Example

abstract-class.php

<?php

abstract class Db
{
    protected $pdo;

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

    abstract function select($table, $fields);
}

class Db_Mysql extends Db
{
    public function select($table, $fields)
    {
        // Build MySQL specific select query
        // then execute it with $this->pdo
    }
}

class Db_Pgsql extends Db
{
    public function select($table, $fields)
    {
        // Build PostgreSQL specific select query
        // then execute it with $this->pdo
    }
}

// Usage:
$db = new Db_Mysql($pdo);

$db->select('users', array('id', 'name'));