Basic knowledge of PHP: classes and objects (1)_PHP tutorial

Class definition:
starts with the keyword class, followed by the class name, which can be any name that is not a PHP reserved word. Followed by a pair of curly braces, which contains the definition of class members and methods. The

pseudo variable $this
can be used when a method is called inside the object . $this is a reference to the calling object (usually the object the method belongs to, but can also be another object if the method is called statically from within a second object).
//??What is the internal function of static call?
Look at an example:
class A
{
function foo( )
{
if (isset($this)) {
echo '$this is defined (';
echo get_class($this);
echo ")n";
                                                                                                                    echo "$this is not defined.n";
A::foo();
}
}
$a = new A();
$a->foo();
A::foo() ;
$b = new B();
$b->bar();
B::bar();

The output result is:

$this is defined (a)
$this is not defined.
$this is defined (b)
$this is not defined.

/*I wish I knew this I can't understand the specific implementation of the example memory and so on. If anyone can explain it clearly, please let me know. The cow head in our dormitory is hibernating now. When he wakes up, I will ask. */

new

To create an instance of an object, you must create a new object and assign it to a variable. A new object is always assigned a value when it is created, unless the object defines a constructor and an exception is thrown on error.
When assigning an already created instance of an object to a new variable, the new variable will access the same instance as if assigned with the object. This behavior is the same as when passing an instance to a function. You can use cloning to create a new instance of an already created object.
(Although these sentences are short, they may not be easy to understand.)
Examples are as follows:
class SimpleClass
{ // Member declaration
public $var = 'a default value';

// Method declaration
public function displayVar() {
echo $this->var;
}}

//See the figure below. 1>new instantiates an object in the heap (heap). 2> Point the pointer $instance to him
$instance = new SimpleClass();

//3> Point the pointer $assigned to the instantiated object in the heap
$ assigned = $instance;
//4>Assign the reference (address) of $instance to $reference
$reference =& $instance;

$ instance->var = '$assigned will have this value';$instance = null; // 5> Interrupt the connection between $instance and the instance in the heap.

var_dump($instance);var_dump($reference);
var_dump($assigned);Output:
NULL
NULLobject(SimpleClass)#1 (1) {
["var"]=>
string(30) "$assigned will have this value"
}
Illustration The whole process:



Class inheritance extends
A class can inherit the methods and members of another class using the extends keyword in the declaration. You cannot extend multiple classes, you can only inherit one base class.

Inherited methods and members can be overridden by redeclaring them with the same name, unless the parent class uses the final keyword when defining the method. Overridden methods or members can be accessed through parent::.

(What is the mechanism of inheritance?)
The source code is as follows:

class ExtendClass extends SimpleClass

{ // Redefine the parent method function displayVar() { echo "Extending classn";
parent::displayVar();
}}
$extended = new ExtendClass();
$extended->displayVar();

Output:

Extending class
a default value

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