const and global in php

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const constant

[Explanation]

1. Must be initialized when defined; 2. Not preceded by Add any modifier; 3. Variable name letters are generally capitalized; 4. Constants can be inherited by subclasses;
5. A constant belongs to a class, not to an object

[Function] When some values ​​are fixed, use const

[Distinguish]What is the difference between constants defined by const and constants defined by define()?

Hope that a member variable will not be modified, for example, pi 3.1415926
Definition: const constant name = value; no $ symbol

Access:Class name::Constant name or interface name::Constant name

And:defined('TEXT' );Check whether a constant with a name exists

<?php　　　　　　　　　　　　　　　　　　　　　
 class A{
 const TAX_RATE=0.08;
 public function payTax($var){
 return $var*A::TAX_RATE;
}
}
$a=new A();
echo $a->payTax(200):
?>

Usage 1: const is used for class membersVariables, once defined, cannot be modified,define It is used for global constants , but cannot be used for the definition of class member variables. const can be used in classes, but define cannot.

Usage 2: Constants defined by const are case-sensitive, and define can specify whether case-sensitivity is achieved through the third parameter (TRUE indicating case-insensitivity). Define a constant at runtime. define('TXE',100,TRUE);

Usage three: const cannot define constants in conditional statements, but the define function can. if($a>10){define('LE','hello');}

global

【Citation: http://www.phptd.com/?action-viewnews-itemid-6147】

1. Super global variable $GLOBALS

There are many super global variables in PHP. The following are all superglobal variables (Superglobal):

$GLOBALS, $_SERVER, $ _GET, $_POST, $_FILES, $_COOKIE, $_SESSION, $_REQUEST, $_ENV.

Official description:

• $GLOBALS — refers to all variables available in the global scope.

• A global combined array containing all variables. The name of the variable is the key of the array.

• The global variables that have appeared can be obtained through the $GLOBALS array.

• In the PHP life cycle, the so-called global variables defined outside the function body cannot be directly obtained inside the function.

##

$foo = "Example content";
test();function test() {    $foo = "local variable";    echo &#39;$foo in current scope: &#39; . $foo . "<br>";    echo &#39;$foo in global scope: &#39; . $GLOBALS["foo"] . "<br>";
}

In the above example, to access external $foo, you must use the $GLOBALS array. This also applies to external global variables that come in through include files.

Global in php also has such a function. The difference between it and $GLOBALS is:

• global generates a pointer to the external variable of the function in the function. Alias ​​variables, not real variables outside the function.

• $GLOBALS[] is indeed called an external variable, and will always be consistent inside and outside the function.

For member variables in the class, functions in the class must use the $this-> method to access, and cannot use the $GLOBALS method:

The role of global is to define Global variables, but this global variable does not apply to the entire website, but to the current page, including all files in include or require.

2. Example explanation

##

function t1() {    global $var1, $var2;    $var2 = &$var1;
}function t2() {    $GLOBALS['var3'] = &$GLOBALS['var1'];
}$var1 = 5;$var2 = $var3 = 0;
t1();print $var2 ."\n";
t2();print $var3 ."\n";

The execution result is:

0

5

Why not 2 5s but 1 0 and What about 1 5?
Modify the example again:

##

function t1() {    global $var1;    $var1 = 2;    unset($var1);
}function t2() {    $GLOBALS['var1'] = 3;    unset($GLOBALS['var1']);
}$var1 = 1;
t1();print $var1 . "\n";
t2();print $var1 . "\n";

执行结果只输入一个2;

1、$GLOBALS是由所有已定义全局变量自动形成的数组。变量名就是该数组的索引。即$GLOBALS['var1']与函数外部的变量$var1是同一个变量，所以将$GLOBALS['var1'] 删除后，该变量已不存在，所有无法输出了。
注：$GLOBALS是自动全局变量。这意味着它在所有的脚本中都有效。在函数或方法中不需要使用 global $GLOBALS; 来访问它。

2、"global $var1;"是产生了函数外部$var1的别名变量"$var1"，它不是真正的函数外部变量，他只存在于函数的内部，所以即使在函数内将别名变量删除也不会影响外面的变量，但是可以修改函数外部变量的值。

或许有的人总想知道这个或那个的区别：在php程序，包括其他程序的学习中，自己动手实验，根据结果加上的思考，有的时候比上网查找可能会来得更快一些，更准确一些。下面我们来讲一下，php在全局范围内访问变量要怎么办？
例一：global定义全局变量。

function test_global() {    global $var1;    $var1 = 'ok';    unset($var1);
}
test_global();$var2 = &$var1;unset($var1);echo $var2;

先不给出结果，自己运行一下程序。函数内部的变量可以访问到了。从结果可以看出，unset只是断开变量名与变量值连接，并没有马上销毁变量的值，而且在函数内部定义的全局变量，实际在外部只是使用了函数内部的别名而已，所以我们在外面依然可以访问$var1。

例二：$GLOBALS在函数内部访问函数外面定义的变量。

$waibu = 'out';function ff() {    echo $GLOBALS['waibu'];
}
ff();

直接在函数内部使用$waibu是会出错的。

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PHP 闭包获取外部变量和global关键字声明变量的区别讲解_php实例

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