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() { $foo = "local variable"; echo '$foo in current scope: ' . $foo . "<br>"; echo '$foo in global scope: ' . $GLOBALS["foo"] . "<br>";
}
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.
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:
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 $var1 . "\n";
执行结果只输入一个2; 先不给出结果，自己运行一下程序。函数内部的变量可以访问到了。从结果可以看出，unset只是断开变量名与变量值连接，并没有马上销毁变量的值，而且在函数内部定义的全局变量，实际在外部只是使用了函数内部的别名而已，所以我们在外面依然可以访问$var1。 直接在函数内部使用$waibu是会出错的。 相关推荐： PHP 闭包获取外部变量和global关键字声明变量的区别讲解_php实例 The above is the detailed content of const and global in php. For more information, please follow other related articles on the PHP Chinese website!
1、$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 $var2;
例二：$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();
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