They are triggered automatically under certain conditions, such as when an object is created, a method is called, or a property is accessed.

1. __construct():

Called when an object is created.

<?php
class Person {
    public $name;
    
    public function __construct($name) {
        $this->name = $name;
    }
}

$person = new Person("John Doe");
echo $person->name; // Output: John Doe
?>

2. __destruct():

 Called when an object is destroyed

<?php
class FileHandler {
    public function __destruct() {
        echo "FileHandler object is being destroyed.\n";
    }
}

$fileHandler = new FileHandler();
// Object is destroyed at the end of the script or when unset
?>

3. __autoload() :

__autoload() method was used to automatically load class files when an undefined class was accessed.

<?php
// The __autoload function
function __autoload($className) {
    include_once $className . '.php';
}

$obj = new MyClass(); // MyClass.php would be loaded automatically
?>

Note: __autoload() method is not present in PHP 8. It was deprecated in PHP 5.3.0 and was officially removed in PHP 7.2.0.

Alternative: Use spl_autoload_register():

<?php
// Define an autoload function
function myAutoloader($className) {
    include_once $className . '.php';  // Adjust the path as needed
}

// Register the autoload function
spl_autoload_register('myAutoloader');

$obj = new MyClass();  // MyClass.php will be loaded automatically
?>

4.  __call() :

Called when an inaccessible or non-existent method is called on an object.

<?php
class MyClass {
    public function __call($method, $args) {
        echo "Method '$method' is not defined.\n";
    }
}

$obj = new MyClass();
$obj->undefinedMethod(); // Output: Method 'undefinedMethod' is not defined.

?>

5. __get():

Called when an inaccessible or non-existent property is accessed.

<?php
class MyClass {
    private $data = ['name' => 'John Doe'];

    public function __get($property) {
        if (isset($this->data[$property])) {
            return $this->data[$property];
        }
        return "Property '$property' not found.";
    }
}

$obj = new MyClass();
echo $obj->name; // Output: John Doe
echo $obj->age;  // Output: Property 'age' not found.

?>

6. __set() : 

Called when an inaccessible or non-existent property is set.

<?php
class MyClass {
    private $data = [];

    public function __set($property, $value) {
        $this->data[$property] = $value;
    }
}

$obj = new MyClass();
$obj->name = 'Jane Doe';  // Sets 'name' property
echo $obj->name;          // Output: Jane Doe

?>

7. __isset() :

Called when isset() or empty() is called on an inaccessible or non-existent property.

<?php
class MyClass {
    private $data = ['name' => 'John'];

    public function __isset($property) {
        return isset($this->data[$property]);
    }
}

$obj = new MyClass();
echo isset($obj->name) ? "Exists" : "Does not exist";  // Output: Exists
echo isset($obj->age) ? "Exists" : "Does not exist";   // Output: Does not exist

?>

8. __unset() :

Called when unset() is used on an inaccessible or non-existent property.

<?php
class MyClass {
    private $data = ['name' => 'John'];

    public function __unset($property) {
        unset($this->data[$property]);
    }
}

$obj = new MyClass();
unset($obj->name);  // Removes 'name' from data

?>

9. _sleep():

called before an object is serialized with serialize(). This method allows you to define which properties should be serialized.

<?php
class MyClass {
    public $name = 'John';
    public $age = 30;

    public function __sleep() {
        return ['name'];  // Only serialize the 'name' property
    }
}

$obj = new MyClass();
$serialized = serialize($obj);
echo $serialized; // Output will contain only 'name' property

?>

10. __wakeup():

Called when an object is unserialized using unserialize(). This is useful for restoring any state after the object is unserialized.

<?php
class MyClass {
    public $name = 'John';
    public $age = 30;

    public function __wakeup() {
        $this->age = 25;  // Reset age after unserialization
    }
}

$obj = new MyClass();
$serialized = serialize($obj);
$unserialized = unserialize($serialized);
echo $unserialized->age;  // Output: 25

?>

11. _toString()

Called when an object is treated as a string, such as when echoing or printing the object.

<?php
class MyClass {
    public $name = 'John';

    public function __toString() {
        return "Name: " . $this->name;
    }
}

$obj = new MyClass();
echo $obj;  // Output: Name: John

?>

12. __invoke()

Called when an object is called as a function.

<?php
class MyClass {
    public function __invoke($param) {
        echo "Invoked with: $param";
    }
}

$obj = new MyClass();
$obj('Hello!');  // Output: Invoked with: Hello!

?>

13. __clone()

Called when an object is cloned using the clone keyword. It allows you to customize the cloning process.

<?php
class MyClass {
    public $name = 'John';

    public function __clone() {
        $this->name = 'Cloned ' . $this->name;
    }
}

$obj = new MyClass();
$clone = clone $obj;
echo $clone->name;  // Output: Cloned John

?>

14. __set_state()

Called when var_export() is used on an object. This method can be used to restore the object's state.

<?php
class MyClass {
    public $name = 'John';

    public static function __set_state($state) {
        $obj = new MyClass();
        $obj->name = $state['name'];
        return $obj;
    }
}

$obj = new MyClass();
$exported = var_export($obj, true);
eval('$restored = ' . $exported . ';');
echo $restored->name;  // Output: John

?>

15. __debugInfo()

Called when var_dump() is used on an object. This method allows you to customize the information displayed during debugging.

<?php
class MyClass {
    public $name = 'John';
    private $secret = 'Hidden';

    public function __debugInfo() {
        return ['name' => $this->name];  // Exclude 'secret' from var_dump
    }
}

$obj = new MyClass();
var_dump($obj);  // Output: object(MyClass)#1 (1) { ["name"]=> string(4) "John" }

?>