Using register_shutdown_function() instead of desctructor in PHP
The usual and might be the most used way of cleaning the object is to use the good old __destruct() magic method in PHP. The magic method proves to be good in most of the cases. But there might be some scenario where even the __destruct() method will fail. For instance, a scenario where your PHP script exceeds the maximum execution time, and terminates thusly. And a fatal error would occur called Maximum execution time of 20 seconds exceeded in - on line XX.
What would you use in such a case? This is where an alternative method register_shutdown_function() can be used.
Basically, you can register a function in register_shutdown_function() that gets executed after script execution finishes or exit() is called.
To understand this, let’s take the following example which I found on php.net.
<?php
class Destruction
{
private $name;
public function __construct($name)
{
$this->name = $name;
register_shutdown_function([&$this, "shutdown"]);
}
public function shutdown()
{
echo 'shutdown: '.$this->name."\n";
}
public function __destruct()
{
echo 'destruct: '.$this->name."\n";
}
}
$a = new Destruction('a: global 1');
function test()
{
$b = new Destruction('b: func 1');
$c = new Destruction('c: func 2');
}
test();
$d = new Destruction('d: global 2');
The output of the above code snippet would be something like below.
shutdown: a: global 1
shutdown: b: func 1
shutdown: c: func 2
shutdown: d: global 2
destruct: b: func 1
destruct: c: func 2
destruct: d: global 2
destruct: a: global 1
We can conclude following things by observing the output.
- The shutdown function is called before the destructor.
- Shutdown functions are called in there “register” order.
- Destructors are always called on script end.
- Destructors are called in order of their “context”: first functions, then global objects.
- Objects in function context are deleted in order as they are set (older objects first).
- Objects in global context are deleted in reverse order (older objects last)
Also, as I mentioned previously, shutdown functions will always gets called no matter the execution time limit gets exceeded for the script. So, it will always be safe to use instead of destructors.
A Parctical Usecase
Many PHP frameworks use this approach for special cases. For instance, Laravel uses register_shutdown_function() here in Illuminate\Console\Scheduling\CallbackEvent in order to remove the mutex for the event no matter what happend to the event. They probably used register_shutdown_function() for the issue with desctructors I’ve mentioned above.
In Closing
The register_shutdown_function() method is clearly not the replacement to the destructors but would turn out to be helpful in certain scenarios. Hope you liked this article and learned a thing or two.
Until next time!
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