php分享三十三:常量

一:常量定义

1:在脚本执行期间该值不能改变(除了所谓的魔术常量,它们其实不是常量)

2:常量默认为大小写敏感

3:命名规则:用正则表达式是这样表达的:[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

在这里,字母指的是 a-z,A-Z,以及从 127 到 255(0x7f-0xff)的 ASCII 字符。

4:

如果使用了一个未定义的常量,PHP 假定想要的是该常量本身的名字,如同用字符串调用它一样(CONSTANT 对应 "CONSTANT")。此时将发出一个 E_NOTICE 级的错误。参见手册中为什么 $foo[bar] 是错误的(除非事先用 define() bar 定义为一个常量)。如果只想检查是否定义了某常量,用 defined() 函数。

settings.php
<?php
// Debug mode
define('DEBUG',false);
?>

test.php
<?php
include('settings.php');

if (DEBUG) {
   // echo some sensitive data.
}
?>

 If for some reason settings.php doesn't get included and the DEBUG constant is not set, PHP will STILL print the sensitive data. The solution is to evaluate it. Like so:

settings.php
<?php
// Debug mode
define('DEBUG',0);
?>

test.php
<?php
include('settings.php');

if (DEBUG == 1) {
   // echo some sensitive data.
}
?>

5:heredoc中的常量不能被解释,如果需要调用,可以把常量封装到一个函数里,返回调用

There is a way to access a constant in heredoc.
Here is the example:
<?php

define("__STR",'constant.string.defined');

function heredocGetConstant($constant) {
  return constant($constant);
}

$heredocGetConstant = 'heredocGetConstant';

$my_string = <<<EOT
This is my constant printed from heredoc: {$heredocGetConstant('__STR')}
EOT;

echo $my_string;

Output:
This is my constant printed from heredoc: constant.string.defined

 

6:It is perfectly valid to use a built-in PHP keyword as a constant name - as long as you the constant() function to retrieve it later:

define('echo', 'My constant value');

echo constant('echo'); // outputs 'My constant value'

7:动态的常量可以用constant()返回

8:常量和(全局)变量在不同的名字空间中。这意味着例如 TRUE 和 $TRUE 是不同的。

 

二:魔术常量(8个)

__FUNCTION__; __FILE__; __LINE__; __NAMESPACE__; __TRAIT__; __CLASS__; __METHOD__; __DIR__;

二:类常量const

1:接口(interface)中也可以定义常量

2:从php5.6开始常量可以用标量表单式了

3:类常量可以被覆盖

it's possible to declare constant in base class, and override it in child, and access to correct value of the const from the static method is possible by 'get_called_class' method:

abstract class dbObject
{    
    const TABLE_NAME='undefined';
    
    public static function GetAll()
    {
        $c = get_called_class();
        return "SELECT * FROM `".$c::TABLE_NAME."`";
    }    
}

class dbPerson extends dbObject
{
    const TABLE_NAME='persons';
}

class dbAdmin extends dbPerson
{
    const TABLE_NAME='admins';
}

echo dbPerson::GetAll()."<br>";//output: "SELECT * FROM `persons`"
echo dbAdmin::GetAll()."<br>";//output: "SELECT * FROM `admins`"

 

二:const和define定义常量的区别

1:和使用 define() 来定义常量相反的是,使用 const 关键字定义常量必须处于最顶端的作用区域,因为用此方法是在编译时定义的。这就意味着不能在函数内,循环内以及 if 语句之内用 const 来定义常量。

2:类常量中不能有变量,define中可以用

The const keyword will not allow anything that may have to be "processed":

$color = "red";
const RED = "This is the color $color"; //Doesn't work
define(strtoupper($color), "This is the color $color") // Works fine

 

posted @ 2016-03-15 10:36  一束光  阅读(296)  评论(0编辑  收藏  举报

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