Here's the approach I take when creating websites in PHP using an MVC/separation of concerns pattern:
The framework I use has three basic pieces:
- Models - PHP Classes. I add methods to them to fetch and save data. Each
model represents a distinct type of entity in the system: users, pages,
blog posts
- Views - Smarty templates. This is where the html lives.
- Controllers - PHP classes. These are the brains of the application. Typically
urls in the site invoke methods of the class. example.com/user/show/1 would
invoke the $user_controller->show(1) method. The controller fetches data out
of the model and gives it to the view.
Each of these pieces has a specific job or "concern". The model's job is to provide a clean interface to the data. Typically the site's data is stored in a SQL database. I add methods to the model for fetching data out and saving data in.
The view's job is to display data. All HTML markup goes in the view. Logic to handle zebra-striping for a table of data goes in the view. Code to handle the format that a date should be displayed in goes in the view. I like using Smarty templates for views because it offers some nice features to handle things like that.
The controller's job is to act as an intermediary between the user, the model, and the view.
Let's look at an example of how these come together and where the benefits lie:
Imagine a simple blog site. The main piece of data is a post. Also, imagine that the site keeps track of the number of times a post is viewed. We'll create a SQL table for that:
posts
id date_created title body hits
Now suppose you would like to show the 5 most popular posts. Here's what you might see in a non MVC application:
$sql = "SELECT * FROM posts ORDER BY hits DESC LIMIT 5";
$result = mysql_query($sql);
while ($row = mysql_fetch_assoc($result)) {
echo "<a href="post.php?id=$row['id']">$row['title']</a><br />";
}
This snippet is pretty straightforward and works well if:
- It is the only place you want to show the most popular posts
- You never want to change how it looks
- You never decide to change what a "popular post" is
Imagine that you want to show the 10 most popular posts on the home page and the 5 most popular in a sidebar on subpages. You now need to either duplicate the code above, or put it in an include file with logic to check where it is being displayed.
What if you want to update the markup for the home page to add a "new-post" class to posts that were created today?
Suppose you decide that a post is popular because it has a lot of comments, not hits. The database will change to reflect this. Now, every place in your application that shows popular posts must be updated to reflect the new logic.
You are starting to see a snowball of complexity form. It's easy to see how things can become increasingly difficult to maintain over the course of a project. Also, consider the complexity when multiple developers are working on a project. Should the designer have to consult with the database developer when adding a class to the output?
Taking an MVC approach and enforcing a separation of concerns within your application can mitigate these issues. Ideally we want to separate it out into three areas:
- data logic
- application logic
- and display logic
Let's see how to do this:
We'll start with the model. We'll have a $post_model class and give it a method called get_popular(). This method will return an array of posts. Additionally we'll give it a parameter to specify the number of posts to return:
post_model.php
class post_model {
public function get_popular($number) {
$sql = "SELECT * FROM posts ORDER BY hits DESC LIMIT $number";
$result = mysql_query($sql);
while($row = mysql_fetch_assoc($result)) {
$array[] = $row;
}
return $array;
}
}
Now for the homepage we have a controller, we'll call it "home". Let's imagine that we have a url routing scheme that invokes our controller when the home page is requested. It's job is to get the popular posts and give them to the correct view:
home_controller.php
class home_controller {
$post_model = new post_model();
$popular_posts = $post_model->get_popular(10);
// This is the smarty syntax for assigning data and displaying
// a template. The important concept is that we are handing over the
// array of popular posts to a template file which will use them
// to generate an html page
$smarty->assign('posts', $popular_posts);
$smarty->view('homepage.tpl');
}
Now let's see what the view would look like:
homepage.tpl
{include file="header.tpl"}
// This loops through the posts we assigned in the controller
{foreach from='posts' item='post'}
<a href="post.php?id={$post.id}">{$post.title}</a>
{/foreach}
{include file="footer.tpl"}
Now we have the basic pieces of our application and can see the separation of concerns.
The model is concerned with getting the data. It knows about the database, it knows about SQL queries and LIMIT statements. It knows that it should hand back a nice array.
The controller knows about the user's request, that they are looking at the homepage. It knows that the homepage should show 10 popular posts. It gets the data from the model and gives it to the view.
The view knows that an array of posts should be displayed as a series of achor tags with break tags after them. It knows that a post has a title and an id. It knows that a post's title should be used for the anchor text and that the posts id should be used in the href. The view also knows that there should be a header and footer shown on the page.
It's also important to mention what each piece doesn't know.
The model doesn't know that the popular posts are shown on the homepage.
The controller and the view don't know that posts are stored in a SQL database.
The controller and the model don't know that every link to a post on the homepage should have a break tag after it.
So, in this state we have established a clear separation of concerns between data logic (the model), application logic (the controller), and display logic (the view). So now what? We took a short simple PHP snippet and broke it into three confusing files. What does this give us?
Let's look at how having a separation of concerns can help us with the issues mentioned above. To reiterate, we want to:
- Show popular posts in a sidebar on subpages
- Highlight new posts with an additional css class
- Change the underlying definition of a "popular post"
To show the popular posts in a sidebar we'll add two files our subpage:
A subpage controller...
subpage_controller.php
class subpage_controller {
$post_model = new post_model();
$popular_posts = $post_model->get_popular(5);
$smarty->assign('posts', $popular_posts);
$smarty->view('subpage.tpl');
}
...and a subpage template:
subpage.tpl
{include file="header.tpl"}
<div id="sidebar">
{foreach from='posts' item='post'}
<a href="post.php?id={$post.id}">{$post.title}</a>
{/foreach}
</div>
{include file="footer.tpl"}
The new subpage controller knows that the subpage should only show 5 popular posts. The subpage view knows that subpages should put the list of posts inside a sidebar div.
Now, on the homepage we want to highlight new posts. We can achieve this by modifying the homepage.tpl.
{include file="header.tpl"}
{foreach from='posts' item='post'}
{if $post.date_created == $smarty.now}
<a class="new-post" href="post.php?id={$post.id}">{$post.title}</a>
{else}
<a href="post.php?id={$post.id}">{$post.title}</a>
{/if}
{/foreach}
{include file="footer.tpl"}
Here the view handles all of the new logic for displaying popular posts. The controller and the model didn't need to know anything about that change. It's purely display logic. The subpage list continues to show up as it did before.
Finally, we'd like to change what a popular post is. Instead of being based on the number of hits a page got, we'd like it to be based on the number of comments a post got. We can apply that change to the model:
post_model.php
class post_model {
public function get_popular($number) {
$sql = "SELECT * , COUNT(comments.id) as comment_count
FROM posts
INNER JOIN comments ON comments.post_id = posts.id
ORDER BY comment_count DESC
LIMIT $number";
$result = mysql_query($sql);
while($row = mysql_fetch_assoc($result)) {
$array[] = $row;
}
return $array;
}
}
We have increased the complexity of the "popular post" logic. However, once we've made this change in the model, in one place, the new logic is applied everywhere. The homepage and the subpage, with no other modifications, will now display popular posts based on comments. Our designer didn't need to be involved in this. The markup is not affected.
Hopefully, this provides a compelling example of how separating the concerns of data logic, application logic, and display logic, can make developing your application easier. Changes in one area tend to have less of an impact on other areas.
Following this convention isn't a magic bullet that will automatically make your code perfect. And you will undoubtedly come across issues where it is far less clear where the separation should be. In the end, it's all about managing complexity within the application.
You should give plenty of thought to how you construct your models. What sort of interfaces will they provide (see Gregory's answer regarding contracts)? What data format does the controller and view expect to work with? Thinking about these things ahead of time will make things easier down the road.
Also, there can be some overhead when starting a project to get all of these pieces working together nicely. There are many frameworks that provide the building blocks for models, controllers, templating engines, url routing, and more. See many other posts on SO for suggestions on PHP MVC frameworks. These frameworks will get you up and running but you as the developer are in charge of managing complexity and enforcing a separation of concerns.
I will also note that the code snippets above are just simplified examples. They may (most likely) have bugs. However, they are very similar in structure to the code I use in my own projects.