Detailed explanation of phpOO static keywords and class constant examples

When I have nothing to do, I just looked for some information about OO in PHP. Access control modifier, self, parent, const, static are the key words. words, arrow operator (also called this in the book...i.e. "->"), range parsing operator (i.e. double colon "::"), but I think this It’s a lot the same as OO in C# language, and it’s easy to understand, but it’s better to take a look and clarify the OO ideas in PHP.

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Declare static class members and methods , so that it does not require an instance of the class. A static member declaration cannot be accessed through an instance of a class object (although a static method can).
Static declaration must follow the visibility declaration. For compatibility with PHP 4, if no visibility is declared, then members and methods will be treated as if they have been declared public.
Since static methods can call non-object instances, the pseudo variable $this cannot be used in methods declared as static.
In fact, the static method calling form is determined at compile time. When using a class name that must be declared, the method is full identification and no inheritance rules apply. This approach is fully valid when using class names that must be declared, and there are no rules for using inheritance.
If self has been declared, then self is interpreted by the class it currently belongs to. Nor does inheritance rules apply. Static properties cannot be accessed from non-static methods through the arrow operator ->., which will generate an E_STRICT level warning.

The code is as follows:

<?php
class Foo
{   public static $my_static=&#39;foo&#39;;
    public function staticValue(){   return self::$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 
// $foo::my_static is not possible
print Bar::$my_static."/n";
$bar = new Bar();
print $bar->fooStatic()."/n";
?>

The code is as follows:

//静态方法实例(Static method example) 
<?php
class Foo
{   public static function aStaticMethod() {    }
}
Foo::aStaticMethod();
?>

You can define constants in each base class to keep it constant. A constant is different from an ordinary variable in that you don't use the $ symbol to declare or use it. Like static members, a constant value cannot be accessed through an instance of an object (use $object::constant instead). The constant value must be a constantexpression, not a variable, a member of a class, The result of a mathematical expression or function call.

<?php
class MyClass
{   const constant = &#39;constant value&#39;;
    function showConstant() {   echo  self::constant."/n";   }
}
echo MyClass::constant."/n";
$class = new MyClass();
$class->showConstant();// echo $class::constant; is not allowed
?>

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