The function names __construct, __destruct (see Constructors and Destructors), __call, __get, __set, __isset, __unset (see Overloading), __sleep, __wakeup, __toString, __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.
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 converted to a string.
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.
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, ...).