Php Destructors

Ok, since my last answer apparently didn't hit the mark, let me try this again. There are plenty of resources and examples on the internet for this topic. Doing a bit of searching and browsing other framework's code and you'll see some pretty good examples...

Don't forget that just because PHP will close resources on termination for you doesn't mean that it's bad to explictly close them when you no longer need them (or good to not close them)... It depends on the use case (is it being used right up to the end, or is there one call early on and then not needed again for the rest of execution)...

Now, we know that __destruct is called when the object is destroyed. Logically, what happens if the object is destroyed? Well, it means it's no longer available. So if it has resources open, doesn't it make sense to close those resources as it's being destroyed? Sure, in the average web page, the page is going to terminate shortly after, so letting PHP close them usually isn't terrible. However, what happens if for some reason the script is long-running? Then you have a resource leak. So why not just close everything when you no longer need it (or considering the scope of the destructor, when it's no longer available)?

Here's some examples in real world frameworks:

Lithium's lithium\net\Socket class
Kohana's Memcached Driver
Joomla's FTP Implementation
Zend Frameworks's SMTP Mail Transport Class
CodeIgniter's TTemplate Class
A Tidy Filter Helper for Cake
A Google-Groups Thread about using Destructors For the Symfony Session Class

The interesting thing is that Kohana keeps track of the tags, so that it can delete by "namespace" later (instead of just clearing the cache). So it uses the destructor to flush those changes to the hard storage.

The CodeIgniter class also does something interesting in that it adds debugging output to the output stream in the destructor. I'm not saying this is good, but it's an example of yet another use...

I personally use destructors whenever I have long running processes on my master controller. In the constructor, I check for a pid file. If that file exists (And its process is still running), I throw an exception. If not, I create a file with the current processes id. Then, in the destructor I remove that file. So it's more about cleaning up after itself than just freeing resources...

There is another handy use to generate HTML page

class HTMLgenerator {
  function __construct() {
    echo "<html><body>";
  }
  function __destruct() {
    echo "</body></html>";
  }
}

With this class, you can write

$html = new HTMLgenerator();
echo "Hello, world!";

And the result is

<html><body>Hello, world!</body></html>

For example:

<?php
class Session
{
    protected $data = array();

    public function __construct()
    {
        // load session data from database or file
    }

    // get and set functions

    public function __destruct()
    {
        // store session data in database or file
    }
};

This is a good why to use destruct. You prevents reading and writing to a session source all the time and do this only at the start and at the end.

I create a php page what will generate a movie information jpg file. This page will have to gather a few information and run inkscape to convert template (an svg file) to a png before converting to jpg. The svg contain relative links to other image which must be a file. So my page download necessary files into a temporary folder, convert the svg file. At the end, the temporary folder must be deleted.

I put the temporary folder deletion into the destructor. Before there can be many reason the page ends unexpected and the only think I can be sure is that destructor will be call when page exit.

Hope this helps.

A destructor is extremely useful if you use a custom database connector/wrapper.

In the constructor, you can pass the connection information. Because you can use a destructor (rather than a finalizer, etc.,) you can rely on that to close the connection for you. It's more of a convenience, but it certainly is useful.

For example, when PHP decides to explicitly "free" the object (i.e., it is no longer used,) it will call the destructor at that time. This is more useful in the scenario I describe as you're not waiting for the garbage collector to run and call the finalizer.

$0.02

Ian

Php Destructors

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