The __toString() method is extremely useful for converting class attribute names and values into common string representations of data (of which there are many choices). I mention this as previous references to __toString() refer only to debugging uses.

I have previously used the __toString() method in the following ways:

 - representing a data-holding object as:
   - XML
   - raw POST data
   - a GET query string
   - header name:value pairs

 - representing a custom mail object as an actual email (headers then body, all correctly represented)

When creating a class, consider what possible standard string representations are available and, of those, which would be the most relevant with respect to the purpose of the class.

Being able to represent data-holding objects in standardised string forms makes it much easier for your internal representations of data to be shared in an interoperable way with other applications.

Please note that as of PHP 8.2 implementing __serialize() has no control over the output of json_encode(). you still have to implement JsonSerializable.

Be very careful to define __set_state() in classes which inherit from a parent using it, as the static __set_state() call will be called for any children.  If you are not careful, you will end up with an object of the wrong type.  Here is an example:

<?php
class A
{
    public $var1; 

    public static function __set_state($an_array)
    {
        $obj = new A;
        $obj->var1 = $an_array['var1'];  
        return $obj;
    }
}

class B extends A {
}

$b = new B;
$b->var1 = 5;

eval('$new_b = ' . var_export($b, true) . ';'); 
var_dump($new_b);
/*
object(A)#2 (1) {
  ["var1"]=>
  int(5)
}
*/
?>

When you use sessions, its very important to keep the sessiondata small, due to low performance with unserialize. Every class shoud extend from this class. The result will be, that no null Values are written to the sessiondata. It will increase performance.

<?
class BaseObject
{
    function __sleep()
    {
        $vars = (array)$this;
        foreach ($vars as $key => $val)
        {
            if (is_null($val))
            {
                unset($vars[$key]);
            }
        }    
        return array_keys($vars);
    }
};
?>

__debugInfo  is also utilised when calling print_r on an object:

$ cat test.php
<?php
class FooQ {

     private $bar = '';

     public function __construct($val) {

         $this->bar = $val;
     }

     public function __debugInfo()
     {
         return ['_bar' => $this->bar];
     }
}
$fooq = new FooQ("q");
print_r ($fooq);

$ php test.php
FooQ Object
(
    [_bar] => q
)
$

http://sandbox.onlinephpfunctions.com/code/4d2cc3648aed58c0dad90c7868173a4775e5ba0c

IMHO a bug or need feature change

providing a object as a array index doesn't try to us __toString() method so some volatile object identifier is used to index the array, which is breaking any persistency. Type hinting solves that, but while other than "string" type hinting doesn't work on ob jects, the automatic conversion to string should be very intuitive.

PS: tried to submit bug, but withot patch the bugs are ignored, unfortunately, I don't C coding

<?php

class shop_product_id {
    
    protected $shop_name;
    protected $product_id;
    
    function __construct($shop_name,$product_id){
        $this->shop_name = $shop_name;
        $this->product_id = $product_id;
    }

    function __toString(){
        return $this->shop_name . ':' . $this->product_id;
    }
}

$shop_name = 'Shop_A';
$product_id = 123;
$demo_id = $shop_name . ':' . $product_id;
$demo_name = 'Some product in shop A';

$all_products = [ $demo_id => $demo_name ];
$pid = new shop_product_id( $shop_name, $product_id );

echo "with type hinting: ";
echo ($demo_name === $all_products[(string)$pid]) ? "ok" : "fail";
echo "\n";

echo "without type hinting: ";
echo ($demo_name === $all_products[$pid]) ?  "ok" : "fail";
echo "\n";

Due to a bug in PHP <= 7.3, overriding the __debugInfo() method from SPL classes is silently ignored.

<?php

class Debuggable extends ArrayObject {
  public function __debugInfo() {
    return ['special' => 'This should show up'];
  }
}

var_dump(new Debuggable());

// Expected output:
// object(Debuggable)#1 (1) {
//   ["special"]=>
//   string(19) "This should show up"
// }

// Actual output:
// object(Debuggable)#1 (1) {
//   ["storage":"ArrayObject":private]=>
//   array(0) {
//   }
// }

?>

Bug report: https://bugs.php.net/bug.php?id=69264

One of the principles of OOP is encapsulation--the idea that an object should handle its own data and no others'.  Asking base classes to take care of subclasses' data, esp considering that a class can't possibly know how many dozens of ways it will be extended, is irresponsible and dangerous.

Consider the following...

<?php
class SomeStupidStorageClass
{
  public function getContents($pos, $len) { ...stuff... }
}

class CryptedStorageClass extends SomeStupidStorageClass
{
  private $decrypted_block;
  public function getContents($pos, $len) { ...decrypt... }
}
?>

If SomeStupidStorageClass decided to serialize its subclasses' data as well as its own, a portion of what was once an encrypted thingie could be stored, in the clear, wherever the thingie was stored.  Obviously, CryptedStorageClass would never have chosen this...but it had to either know how to serialize its parent class's data without calling parent::_sleep(), or let the base class do what it wanted to.

Considering encapsulation again, no class should have to know how the parent handles its own private data.  And it certainly shouldn't have to worry that users will find a way to break access controls in the name of convenience.

If a class wants both to have private/protected data and to survive serialization, it should have its own __sleep() method which asks the parent to report its own fields and then adds to the list if applicable.  Like so....

<?php

class BetterClass
{
  private $content;

  public function __sleep()
  {
    return array('basedata1', 'basedata2');
  }

  public function getContents() { ...stuff... }
}

class BetterDerivedClass extends BetterClass
{
  private $decrypted_block;

  public function __sleep()
  {
    return parent::__sleep();
  }

  public function getContents() { ...decrypt... }
}

?>

The derived class has better control over its data, and we don't have to worry about something being stored that shouldn't be.

If you use the Magical Method '__set()', be shure that the call of
<?php
$myobject->test['myarray'] = 'data';
?>
will not appear!

For that u have to do it the fine way if you want to use __set Method ;)
<?php
$myobject->test = array('myarray' => 'data');
?>

If a Variable is already set, the __set Magic Method already wont appear!

My first solution was to use a Caller Class.
With that, i ever knew which Module i currently use!
But who needs it... :]
There are quiet better solutions for this...
Here's the Code:

<?php
class Caller {
    public $caller;
    public $module;

    function __call($funcname, $args = array()) {
        $this->setModuleInformation();

        if (is_object($this->caller) && function_exists('call_user_func_array'))
            $return = call_user_func_array(array(&$this->caller, $funcname), $args);
        else
            trigger_error("Call to Function with call_user_func_array failed", E_USER_ERROR);
        
        $this->unsetModuleInformation();
        return $return;
    }

    function __construct($callerClassName = false, $callerModuleName = 'Webboard') {
        if ($callerClassName == false)
            trigger_error('No Classname', E_USER_ERROR);

        $this->module = $callerModuleName;

        if (class_exists($callerClassName))
            $this->caller = new $callerClassName();
        else
            trigger_error('Class not exists: \''.$callerClassName.'\'', E_USER_ERROR);

        if (is_object($this->caller))
        {
            $this->setModuleInformation();
            if (method_exists($this->caller, '__init'))
                $this->caller->__init();
            $this->unsetModuleInformation();
        }
        else
            trigger_error('Caller is no object!', E_USER_ERROR);
    }

    function __destruct() {
        $this->setModuleInformation();
        if (method_exists($this->caller, '__deinit'))
            $this->caller->__deinit();
        $this->unsetModuleInformation();
    }

    function __isset($isset) {
        $this->setModuleInformation();
        if (is_object($this->caller))
            $return = isset($this->caller->{$isset});
        else
            trigger_error('Caller is no object!', E_USER_ERROR);
        $this->unsetModuleInformation();
        return $return;
    }

    function __unset($unset) {
        $this->setModuleInformation();
        if (is_object($this->caller)) {
            if (isset($this->caller->{$unset}))
                unset($this->caller->{$unset});
        }
        else
            trigger_error('Caller is no object!', E_USER_ERROR);
        $this->unsetModuleInformation();
    }

    function __set($set, $val) {
        $this->setModuleInformation();
        if (is_object($this->caller))
            $this->caller->{$set} = $val;
        else
            trigger_error('Caller is no object!', E_USER_ERROR);
        $this->unsetModuleInformation();
    }

    function __get($get) {
        $this->setModuleInformation();
        if (is_object($this->caller)) {
            if (isset($this->caller->{$get}))
                $return = $this->caller->{$get};
            else
                $return = false;
        }
        else
            trigger_error('Caller is no object!', E_USER_ERROR);
        $this->unsetModuleInformation();
        return $return;
    }
    
    function setModuleInformation() {
        $this->caller->module = $this->module;
    }

    function unsetModuleInformation() {
        $this->caller->module = NULL;
    }
}

// Well this can be a Config Class?
class Config {
    public $module;

    public $test;

    function __construct()
    {
        print('Constructor will have no Module Information... Use __init() instead!<br />');
        print('--> '.print_r($this->module, 1).' <--');
        print('<br />');
        print('<br />');
        $this->test = '123';
    }
    
    function __init()
    {
        print('Using of __init()!<br />');
        print('--> '.print_r($this->module, 1).' <--');
        print('<br />');
        print('<br />');
    }
    
    function testFunction($test = false)
    {
        if ($test != false)
            $this->test = $test;
    }
}

echo('<pre>');
$wow = new Caller('Config', 'Guestbook');
print_r($wow->test);
print('<br />');
print('<br />');
$wow->test = '456';
print_r($wow->test);
print('<br />');
print('<br />');
$wow->testFunction('789');
print_r($wow->test);
print('<br />');
print('<br />');
print_r($wow->module);
echo('</pre>');
?>

Outputs something Like:

Constructor will have no Module Information... Use __init() instead!
-->  <--

Using of __init()!
--> Guestbook <--

123

456

789

Guestbook

Intriguing what happens when __sleep() and __wakeup() and sessions() are mixed. I had a hunch that, as session data is serialized, __sleep would be called when an object, or whatever, is stored in _SESSION. true. The same hunch applied when session_start() was called. Would __wakeup() be called? True. Very helpful, specifically as I'm building massive objects (well, lots of simple objects stored in sessions), and need lots of automated tasks (potentially) reloaded at "wakeup" time. (for instance, restarting a database session/connection).