The function names __construct(), __destruct(), __call(), __callStatic(), __get(), __set(), __isset(), __unset(), __sleep(), __wakeup(), __toString(), __invoke(), __set_state() and __clone() are magical in PHP classes. You cannot have functions with these names in any of your classes unless you want the magic functionality associated with them.
PHP reserves all function names starting with __ as magical. It is recommended that you do not use function names with __ in PHP unless you want some documented magic functionality.
serialize() checks if your class has a function with
the magic name __sleep(). If so, that function is
executed prior to any serialization. It can clean up the object
and is supposed to return an array with the names of all variables
of that object that should be serialized.
If the method doesn't return anything then NULL is serialized and
E_NOTICE is issued.
Note:
It is not possible for __sleep() to return names of private properties in parent classes. Doing this will result in an
E_NOTICElevel error. Instead you may use the Serializable interface.
The intended use of __sleep() is to commit pending data or perform similar cleanup tasks. Also, the function is useful if you have very large objects which do not need to be saved completely.
Conversely, unserialize() checks for the presence of a function with the magic name __wakeup(). If present, this function can reconstruct any resources that the object may have.
The intended use of __wakeup() is to reestablish any database connections that may have been lost during serialization and perform other reinitialization tasks.
Example #1 Sleep and wakeup
<?php
class Connection
{
protected $link;
private $server, $username, $password, $db;
public function __construct($server, $username, $password, $db)
{
$this->server = $server;
$this->username = $username;
$this->password = $password;
$this->db = $db;
$this->connect();
}
private function connect()
{
$this->link = mysql_connect($this->server, $this->username, $this->password);
mysql_select_db($this->db, $this->link);
}
public function __sleep()
{
return array('server', 'username', 'password', 'db');
}
public function __wakeup()
{
$this->connect();
}
}
?>
The __toString() method allows a class to decide
how it will react when it is treated like a string. For example,
what echo $obj; will print. This method must
return a string, as otherwise a fatal E_RECOVERABLE_ERROR
level error is emitted.
You cannot throw an exception from within a __toString() method. Doing so will result in a fatal error.
Example #2 Simple example
<?php
// Declare a simple class
class TestClass
{
public $foo;
public function __construct($foo)
{
$this->foo = $foo;
}
public function __toString()
{
return $this->foo;
}
}
$class = new TestClass('Hello');
echo $class;
?>
The above example will output:
Hello
It is worth noting that before PHP 5.2.0 the __toString()
method was only called when it was directly combined with
echo or print.
Since PHP 5.2.0, it is called in any string context (e.g. in
printf() with %s modifier) but not
in other types contexts (e.g. with %d modifier).
Since PHP 5.2.0, converting objects without __toString()
method to string would cause E_RECOVERABLE_ERROR.
The __invoke() method is called when a script tries to call an object as a function.
Note:
This feature is available since PHP 5.3.0.
Example #3 Using __invoke()
<?php
class CallableClass
{
public function __invoke($x)
{
var_dump($x);
}
}
$obj = new CallableClass;
$obj(5);
var_dump(is_callable($obj));
?>
The above example will output:
int(5) bool(true)
$properties
)This static method is called for classes exported by var_export() since PHP 5.1.0.
The only parameter of this method is an array containing exported properties in the form array('property' => value, ...).
Example #4 Using __set_state() (since PHP 5.1.0)
<?php
class A
{
public $var1;
public $var2;
public static function __set_state($an_array) // As of PHP 5.1.0
{
$obj = new A;
$obj->var1 = $an_array['var1'];
$obj->var2 = $an_array['var2'];
return $obj;
}
}
$a = new A;
$a->var1 = 5;
$a->var2 = 'foo';
eval('$b = ' . var_export($a, true) . ';'); // $b = A::__set_state(array(
// 'var1' => 5,
// 'var2' => 'foo',
// ));
var_dump($b);
?>
The above example will output:
object(A)#2 (2) {
["var1"]=>
int(5)
["var2"]=>
string(3) "foo"
}