register_shutdown_function

If your script exceeds the maximum execution time, and terminates thusly:

Fatal error: Maximum execution time of 20 seconds exceeded in - on line 12

The registered shutdown functions will still be executed.

I figured it was important that this be made clear!

If you want to do something with files in function, that registered in register_shutdown_function(), use ABSOLUTE paths to files instead of relative. Because when script processing is complete current working directory chages to ServerRoot (see httpd.conf)

warning: in addition to SIGTERM and SIGKILL, the shutdown functions won't run in response to SIGINT either. (observed on php 7.1.16 on windows 7 SP1 x64 + cygwin  and php 7.2.15 on Ubuntu 18.04)

You may get the idea to call debug_backtrace or debug_print_backtrace from inside a shutdown function, to trace where a fatal error occurred. Unfortunately, these functions will not work inside a shutdown function.

A lot of useful services may be delegated to this useful trigger.
It is very effective because it is executed at the end of the script but before any object destruction, so all instantiations are still alive.

Here's a simple shutdown events manager class which allows to manage either functions or static/dynamic methods, with an indefinite number of arguments without using any reflection, availing on a internal handling through func_get_args() and call_user_func_array() specific functions:

<?php
// managing the shutdown callback events:
class shutdownScheduler {
    private $callbacks; // array to store user callbacks
    
    public function __construct() {
        $this->callbacks = array();
        register_shutdown_function(array($this, 'callRegisteredShutdown'));
    }
    public function registerShutdownEvent() {
        $callback = func_get_args();
        
        if (empty($callback)) {
            trigger_error('No callback passed to '.__FUNCTION__.' method', E_USER_ERROR);
            return false;
        }
        if (!is_callable($callback[0])) {
            trigger_error('Invalid callback passed to the '.__FUNCTION__.' method', E_USER_ERROR);
            return false;
        }
        $this->callbacks[] = $callback;
        return true;
    }
    public function callRegisteredShutdown() {
        foreach ($this->callbacks as $arguments) {
            $callback = array_shift($arguments);
            call_user_func_array($callback, $arguments);
        }
    }
    // test methods:
    public function dynamicTest() {
        echo '_REQUEST array is '.count($_REQUEST).' elements long.<br />';
    }
    public static function staticTest() {
        echo '_SERVER array is '.count($_SERVER).' elements long.<br />';
    }
}
?>

A simple application:

<?php
// a generic function
function say($a = 'a generic greeting', $b = '') {
    echo "Saying {$a} {$b}<br />";
}

$scheduler = new shutdownScheduler();

// schedule a global scope function:
$scheduler->registerShutdownEvent('say', 'hello!');

// try to schedule a dyamic method:
$scheduler->registerShutdownEvent(array($scheduler, 'dynamicTest'));
// try with a static call:
$scheduler->registerShutdownEvent('scheduler::staticTest');

?>

It is easy to guess how to extend this example in a more complex context in which user defined functions and methods should be handled according to the priority depending on specific variables.

Hope it may help somebody.
Happy coding!

You definitely need to be careful about using relative paths in after the shutdown function has been called, but the current working directory doesn't (necessarily) get changed to the web server's ServerRoot - I've tested on two different servers and they both have their CWD changed to '/' (which isn't the ServerRoot).

This demonstrates the behaviour:

<?php
function echocwd() { echo 'cwd: ', getcwd(), "\n"; }

register_shutdown_function('echocwd');
echocwd() and exit;
?>

Outputs:

cwd: /path/to/my/site/docroot/test
cwd: /

NB: CLI scripts are unaffected, and keep their CWD as the directory the script was called from.

Contrary to the the note that "headers are always sent" and some of the comments below - You CAN use header() inside of a shutdown function as you would anywhere else; when headers_sent() is false. You can do custom fatal handling this way:

<?php
ini_set('display_errors',0);
register_shutdown_function('shutdown');

$obj = new stdClass();
$obj->method();

function shutdown()
{
  if(!is_null($e = error_get_last()))
  {
    header('content-type: text/plain');
    print "this is not html:\n\n". print_r($e,true);
  }
}
?>

I have discovered a change in behavior from PHP 5.0.4 to PHP 5.1.2 when using a shutdown function in conjunction with an output buffering callback.

In PHP 5.0.4 (and earlier versions I believe) the shutdown function is called after the output buffering callback.

In PHP 5.1.2 (not sure when the change occurred) the shutdown function is called before the output buffering callback.

Test code:
<?php
function ob_callback($buf) {
    $buf .= '<li>' . __FUNCTION__ .'</li>';
    return $buf;
}

function shutdown_func() {
    echo '<li>' . __FUNCTION__ .'</li>';
}

ob_start('ob_callback');
register_shutdown_function('shutdown_func');
echo '<ol>';
?>

PHP 5.0.4:

1. ob_callback
2. shutdown_func

PHP 5.1.2:

1. shutdown_func
2. ob_callback

Something found out during testing:

the ini auto_append_file will be included BEFORE the registered function(s) will be called.

When using php-fpm, fastcgi_finish_request() should be used instead of register_shutdown_function() and exit()

For example, under nginx and php-fpm 5.3+, this will make browsers wait 10 seconds to show output:

<?php
    echo "You have to wait 10 seconds to see this.<br>";
    register_shutdown_function('shutdown');
    exit;
    function shutdown(){
        sleep(10);
        echo "Because exit() doesn't terminate php-fpm calls immediately.<br>";
    }
?>

This doesn't:

<?php
    echo "You can see this from the browser immediately.<br>";
    fastcgi_finish_request();
    sleep(10);
    echo "You can't see this form the browser.";
?>

I performed two tests on the register_shutdown_function() to see under what conditions it was called, and if a can call a static method from a class. Here are the results:

<?php
/**
 * Tests the shutdown function being able to call a static methods
 */
class Shutdown
{
    public static function Method ($mixed = 0)
    {
        // we need absolute
        $ap = dirname (__FILE__);
        $mixed = time () . " - $mixed\n";
        file_put_contents ("$ap/shutdown.log", $mixed, FILE_APPEND);
    }
}
// 3. Throw an exception
register_shutdown_function (array ('Shutdown', 'Method'), 'throw');
throw new Exception ('bla bla');

// 2. Use the exit command
//register_shutdown_function (array ('Shutdown', 'Method'), 'exit');
//exit ('exiting here...')

// 1. Exit normally
//register_shutdown_function (array ('Shutdown', 'Method'));
?>

To test simply leave one of the three test lines uncommented and execute. Executing bottom-up yielded:

1138382480 - 0
1138382503 - exit
1138382564 - throw

HTH

If you need the old (<4.1) behavior of register_shutdown_function you can achieve the same with "Connection: close" and "Content-Length: xxxx" headers if you know the exact size of the sent data (which can be easily caught with output buffering).
An example:
<?php
header("Connection: close");
ob_start();
phpinfo();
$size=ob_get_length(); 
header("Content-Length: $size");
ob_end_flush();
flush();
sleep(13);
error_log("do something in the background");
?>

The same will work with registered functions.
According to http spec, browsers should close the connection when they got the amount of data specified in Content-Length header. At least it works fine for me in IE6 and Opera7.

Here is a great example of using shutdown.

I am using a database class for 20 years now. Starting with mysql_connect of course. then transformed it to mysqli as time went by. It was a real transition at some point.

It recently came to my attention that die() and exit() will not handle mysql_close() $con->close properly. __destruct might not be called.

so a good way around this is register the shutdown function so mysql will always close.

    function __destruct(){
        $this->close();
    }
    function close() {
        // this function closes the database
        if($this->connected){
            if($this->use_mysqli){
                $this->connected = false;
                return $this->con->close();
            } else {
                $this->connected = false;
                return mysql_close($this->con);
            }
        }
    }

    function __construct($host, $user, $pass, $db){
        global $_CONFIG;
        if(isset($_CONFIG['use_mysql']) && $_CONFIG['use_mysql']){
            $this->use_mysqli = false;    
        } else {
            $this->use_mysqli = function_exists("mysqli_connect");
        }
        $this->database($host, $user, $pass, $db);
        register_shutdown_function([$this, 'close']);
    }

By Jacob Slomp

> Shutdown functions run separately from the time tracked by max_execution_time. That means even if a process is terminated for running too long, shutdown functions will still be called. Additionally, if the max_execution_time runs out while a shutdown function is running it will not be terminated.

This does not appear to be true in our testing, specifically "Additionally, if the max_execution_time runs out while a shutdown function is running it will not be terminated".  For example:

<?php
set_time_limit(5);

register_shutdown_function(function() {
    $start = time();
    for($i = 0; $i < 40; $i++) {
        echo "Run 1: $i\n";
        while(1) {
            $elapsed = time() - $start;
            if($elapsed > $i) {
                break;
            }
        }
    }
});

?>

This will print out:

Run 1: 0
Run 1: 1
Run 1: 2
Run 1: 3
Run 1: 4
Run 1: 5

Then it will die with:

Execution took longer than 5 seconds, sent SIGTERM and terminated

If you register multiple shutdown functions and an earlier one exceeds the execution time the later ones will _not_ be run. 

If you need your shutdowns to have unlimited time (like the docs suggest it works) one solution might be to register a shutdown function like this early in your code:

<?php
register_shutdown_function(function() {
    set_time_limit(0);
});
?>

So that you give yourself unlimited time in your subsequent shutdown functions.

See an example here: https://www.tehplayground.com/wJLtMi3Z5c1sTi9Y (Note, 5 seconds is the max you can set on this website. If you remove the first shutdown function you it will be killed after 3 seconds).

I wanted to set the exit code for a CLI application from a shutdown function by using a class property, 

Unfortunataly (as per manual)  "If you call exit() within one registered shutdown function, processing will stop completely and no other registered shutdown functions will be called."  

As a result if I call exit in my shutdown function I will break other shutdown functions (like one that logs fatal errors to syslog)  

However! (as per manual)  "Multiple calls to register_shutdown_function() can be made, and each will be called in the same order as they were registered."

As luck would have it you are also able to register a shutdown function from within a shutdown function (at least in PHP 7.0.15 and 5.6.30) 

in other words if you register a shutdown function inside a shutdown function it is appended to the shutdown function queue.

<?php 
class SomeApplication{
    private $exitCode = null;
        
    public function __construct(){
        register_shutdown_function(function(){
        echo "some registered shutdown function";
            register_shutdown_function(function(){
                echo "last registered shutdown function";
                // one might even consider another register shutdown_function if one expects other shutdown functions to do the same...  
                exit($this->exitCode === null ? 0 : $this->exitCode);
            });
        });
    }
}
?>