适配器模式，可以将截然不同的函数接口封装成统一的API

例如，PHP的数据库操作有mysql, mysqli, pdo 3种，可以用适配器模式统一成一致。类似的情景还有caceh适配器，将memcache, redis, file, apc等不同的缓存函数统一成一致。

//IDatabase.php 统一的接口
<?php

namespace App\Database;

interface IDatabase
{
    function connect($host, $name, $password, $database);
    
    function query($sql);
    
    function close();
}

//MySQL.php MySQL类
<?php

namespace App\Database;

class MySQL implements IDatabase
{
    protected $conn;

    function connect($host, $name, $password, $database)
    {
        $conn = mysql_connect($host, $name, $password);
        mysql_select_db($database, $conn);
        $this->conn = $conn;
    }

    function query($sql)
    {
        return mysql_query($sql);
    }

    function close()
    {
        mysql_close($this->conn);
    }

}

//MySQLi.php MySQLi类
<?php

namespace App\Database;

class MySQLi implements IDatabase
{
    protected $conn;
    
    function connect($host, $name, $password, $database)
    {
        $conn = mysqli_connect($host, $name, $password, $database);
        
        return $this->conn = $conn;
    }

    function query($sql)
    {
        return mysqli_query($this->conn, $sql);
    }

    function close()
    {
        mysqli_close($this->conn);
    }

}

//PDO.php PDO类
<?php

namespace App\Database;

class PDO implements IDatabase
{
    protected $conn;
    
    function connect($host, $name, $password, $database)
    {
        $dsn = "mysql:host=$host;dbname=$database";
        $conn = new \PDO($dsn, $name, $password);

        return $this->conn = $conn;
    }

    function query($sql)
    {
        return $this->conn->query($sql);
    }

    function close()
    {
        unset($this->conn);
    }

}

//index.php 使用示例
<?php

use App\Database\MySQL;

$conn = new MySQL();
$conn->connect('127.0.0.1', 'root', '', 'test');
$res = $conn->query("show databases");
$conn->close();