What's the best way to abstract the database from a PHP application?

Question

My question is how does one abstract a database connection from the model layer of an application? The primary concern is to be able to easily change from different types of databases. Maybe you start with a flat file, comma-delimited database. Then you want to move to a SQL database. Then later you decide an LDAP implementation would be better. How can a person easily plan for something like this?

For a simple example, let's say you have a user with a first name, last name, and email. A very simple PHP class representing it might look like this (please ignore the problems with public instance variables):

<?php

class User {
  public $first;
  public $last;
  public $email;
}

?>

I have frequently seen where people have a DAO class which has the SQL embedded in it as such:

<?php

class UserDAO {
  public $id;
  public $fist;
  public $last;
  public $email;

  public function create( &$db ) {
    $sql = "INSERT INTO user VALUES( '$first', '$last', '$email' )";
    $db->query( $sql );
  }
}

?>

My problem with strategies like this is when you want to change your database, you have to change every DAO class' create, update, load, delete functions to deal with your new type of database. Even if you have a program to auto-generate them for you (which I am not particularly a fan of), you would have to edit this program to make it work now.

What are your suggestions for how to handle this?

My current idea is to create a super class for DAO objects with its own create, delete, update, load functions. However, these functions would take arrays of the attributes of the DAO and generate the query itself. In this manner, the only SQL is in the SuperDAO class rather than being scattered about several classes. Then if you wanted to change your database layer, you would only have to change how the SuperDAO class generates the queries. Advantages? Disadvantages? Foreseeable problems? The good, the bad, and the ugly?

Answer 1

You should look into the PDO library.

PDO provides a data-access abstraction layer, which means that, regardless of which database you're using, you use the same functions to issue queries and fetch data.

PDO does not provide a database abstraction; it doesn't rewrite SQL or emulate missing features. You should use a full-blown abstraction layer if you need that facility.

Answer 2

It sounds good in theory but in all likelyhood YAGNI.

You would be better off using an SQL library such as PDO and not worrying about LDAP until you get there.

Answer 3

Using an ORM is usually the preferred way of abstracting the database. An incomplete list of PHP implementations is available on Wikipedia.

Answer 4

You can use various frameworks such as PDO, PEAR::MDB2 or Zend_Db, but to be honest in 12 years of PHP development, I've never had to transition from one type of data storage infrastructure to another.

Its exceedingly rare to even go from something quite similar like Sqlite, to MySQL. If you did do more than that, you'd have far larger problems anyway.

Answer 5

Generally speaking, if you're going to the trouble of using a database then your application will benefit by using features specific to a "brand" of database, and will be a more solid app for it.

It is very rare to move from one database system to another. The only time you might realistically consider that aa feature worth implementing is if you're writing some kind of loosely coupled system or framework intended for mass consumption (like Zend Framework, or Django).

Answer 6

Checkout this thread about ORMs for PHP: http://stackoverflow.com/questions/108699/good-php-orm-library

Answer 7

I always liked using ADOdb. From what I've seen, it looks like it's capable of switching between vastly different platforms.

http://adodb.sf.net

Answer 8

Axon ORM automatically detects changes in your schema without requiring you to rebuild your code.