Discuss in detail the usage of public, private, protected, abstract and other keywords in PHP

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Commonly used keywords in PHP

contains many restrictions on functions and classes in PHP Commonly used keywords usually include abstract, final, interface, public, protected, private, static, etc. Below we will analyze and sort out their usage.

Keywords for variables and methods public, private, protected

public has the greatest authority and can be used by subclasses or supported When called after instantiation,

protected means protected, and access permissions can only be accessed in subclasses and this class.

private means private, only It may be that the keyword static

<?php
//
/**
* Define MyClass
*/
class MyClass
{
 public $public = &#39;Public&#39;;
 protected $protected = &#39;Protected&#39;;
 private $private = &#39;Private&#39;;
 public function printHello()
 {
 echo $this->public;
 echo $this->protected;
 echo $this->private;
 }
 protected function pro_test(){
 var_dump(1);
 }
}
$obj = new MyClass();
echo $obj->public; // 这行能被正常执行
//echo $obj->protected; // 这行会产生一个致命错误
//echo $obj->private; // 这行也会产生一个致命错误
$obj->printHello(); // 输出 Public、Protected 和 Private
$obj->pro_test();//直接报错
?>

## variables and methods can be accessed in the current class

The function of static is to enable values ​​or methods to be called in a class without instantiation. At the same time, static-modified variables have the function of storing values. For example, if we do not use static, the result of running is as follows:

<?php
function test(){
 $var=1;
 echo $var."</br>";
 $var++;
}
test();// 1
test();// 1
test();// 1
?>

If we add static to the variable, it will become like this

<?php
function test(){
 static $var=1;
 echo $var."</br>";
 $var++;
}
test();// 1
test();// 2
test();// 3
?>

Here You may not be able to realize the benefits of doing this in PHP, so let's first assume that the reader is also familiar with JS. There is no static keyword in JS, so if we want to implement a program that can save the results of each program operation as We need to write a program based on the next operation like this.

var glo=0;
 function test(){
 glo++;
 document.writeln(glo);
 }
 test();
 test();
 test();

This will leak glo into global variables. If more variables are generated, it will lead to memory leaks (memory leaks refer to Variables occupy too much memory space and will not be released)

<script>
 var glo=0;
 function test(){
 glo++;
 document.writeln(glo);
 }
 test();// 1
 test();// 2
 test();// 3 
</script>

So compared to languages ​​that do not define static, it has the advantage of retaining variables. It does not leak memory, and it is not easy to cause global variables to be misused (because the scope is not global)

$age=0;
$age++;
function test1() {
 static $age = 100;
 $age++;
 echo $age."</br>";
}
function test2() {
 static $age = 1000;
 $age++;
 echo $age."</br>";
}
test1(); // 101
test2(); // 1001

The key to class and method The word final

final can only be used to modify class and function. After using final, it cannot be inherited. For example, the following code will directly report an error

class BaseClass {
 public $public = 'Public';
 function test() {
 echo "BaseClass::test() called\n";
 }
 final public function moreTesting() {
 echo "BaseClass::moreTesting() called\n";
 }
}
class ChildClass extends BaseClass {
 public function moreTesting() {
 echo "ChildClass::moreTesting() called\n";
 }
}

Special keywords interface,abstract##The meaning of interface is to standardize the programming style. Imagine if it is implemented If we have an interface, then when we use this interface class, we must implement the methods inside, which plays a role in unified naming.

Class can inherit multiple interfaces. Single inheritance between interfaces is achieved through extends. The relationship between class and interface is established through implements

Sample code:

<?php
interface testA{
 function funcA();
}
interface testB{
 function funcB();
}
interface testC extends testA {
 function funcC();
}
class run implements testC ,testB {
 public function funcA()
 {
 // TODO: Implement funcA() method.
 }
 public function funcB()
 {
 // TODO: Implement funcB() method.
 }
 public function funcC()
 {
 // TODO: Implement funcC() method.
 }
}
?>

The function of abstract is actually the same as that of interface, but all methods in interface must be implemented, but in classes modified by abstract, There can be one or more abstract modification methods, so we can understand that interface is a special case of abstract (when all methods are abstract methods, they must be implemented). Abstract has the following characteristics:

1. As long as at least one method in the class uses the abstract keyword, then the class is abstract, and the corresponding keyword must be added

2. Abstract Methods only have the declaration part of the method and no method body.

But in my opinion, abstract has several scenarios like this in actual applications

1. Standardize the naming rules of public parts when multi-person programming (without any explanation, the principle Same as interface)

2. To prevent the parent from being instantiated directly, use the

style code as follows:

<?php
abstract class shopping
{
 public function buy()
 {
 echo "buy";
 }
 public function loan()
 {
 echo "loan";
 }
}
class leslieBuy extends shopping
{
}
//$test1=new shopping;//直接语法错误
$leslieSie = new leslieBuy;
$leslieSie->loan();//打印出loan
?>

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