php的static关键字

Static Keyword

Tip

    This page describes the use of the static keyword to    define static methods and properties. static can also    be used to    define static variables    and for    late static bindings.    Please refer to those pages for information on those meanings of    static.  

   Declaring class properties or methods as static makes them accessible   without needing an instantiation of the class. A property declared as   static can not be accessed with an instantiated class object (though   a static method can). 

   For compatibility with PHP 4, if no visibility   declaration is used, then the property or method will be treated   as if it was declared as public. 

   Because static methods are callable without an instance of   the object created, the pseudo-variable $this is   not available inside the method declared as static. 

   Static properties cannot be accessed through the object using the arrow   operator ->. 

   Calling non-static methods statically generates an E_STRICT level warning. 

   Like any other PHP static variable, static properties may only be   initialized using a literal or constant; expressions are not   allowed. So while you may initialize a static property to an   integer or array (for instance), you may not initialize it to   another variable, to a function return value, or to an object. 

   As of PHP 5.3.0, it's possible to reference the class using a variable.   The variable's value can not be a keyword (e.g. self,   parent and static). 

Example #1 Static property example

<?php
class Foo
{
    public static $my_static = 'foo';

    public function staticValue() {
        return self::$my_static;
    }
}

class Bar extends Foo
{
    public function fooStatic() {
        return parent::$my_static;
    }
}


print Foo::$my_static . "\n";

$foo = new Foo();
print $foo->staticValue() . "\n";
print $foo->my_static . "\n";      // Undefined "Property" my_static

print $foo::$my_static . "\n";
$classname = 'Foo';
print $classname::$my_static . "\n"; // As of PHP 5.3.0

print Bar::$my_static . "\n";
$bar = new Bar();
print $bar->fooStatic() . "\n";
?>

Example #2 Static method example

<?php
class Foo {
    public static function aStaticMethod() {
        // ...
    }
}

Foo::aStaticMethod();
$classname = 'Foo';
$classname::aStaticMethod(); // As of PHP 5.3.0
?>