PHP 7: PHP 变量和常量的定义
本章说说变量的定义。
如果对于变量和常量的定义，你会注意几个方面呢？你可能会想到：
如何定义变量，它和C# 等语言有什么不同呢？ 变量区分大小写吗？ PHP的变量还有其他重要的吗？
常量和变量的定义一样吗？ 分别讲述吧。
1.如何定义变量，它和C# 等语言有什么不同呢？
PHP 中的变量用一个 美元符号后面跟变量名来表示。变量名是 区分大小写的。例如:
$var = ' Jim ' ;
$VAR = ' Kimi;
echo "$var,$VAR";//输出“Jim,Kimi"
?>
2. 变量区分大小写吗？
如 1里说的，区分大小写。
注意，需要说明的一点是自PHP4以来，引入了引用赋值的概念，其实和多数语言的引用类似，不过我觉得最类似的是C/C++.因为它也用到了"&"符号。例如：
1 php
2 $foo = ' Bob ' ; // 赋值'Bob'给foo
3 $bar = & $foo ; // 通过$bar引用.注意&符号
4 $bar = " My name is $bar " ; // 修改 $bar
5 echo $bar ;
6 echo $foo ; // $foo 也修改了.
7 ?>
3. PHP其他重要点
预定义变量
预定义变量在PHP是一个重要的概念。 PHP 提供了大量的预定义变量。由于许多这些变量依赖于运行的服务器的版本和设置，及其它因素，所以并没有详细的说明文档。一些预定义变量在 PHP 以命令行形式运行时并不生效。
需要注意的是 在PHP 4.2.0 以及后续版本中，PHP 指令 register_globals 的默认值为 off。这是 PHP 的一个主要变化。让 register_globals 的值为 off 将影响到预定义变量集在全局范围内的有效性。例如，为了得到 DOCUMENT_ROOT 的值，将必须使用 $_SERVER['DOCUMENT_ROOT'] 代替 $DOCUMENT_ROOT，又如，使用 $_GET['id'] 来代替 $id 从 URL http://www.example.com/test.php?id=3 中获取 id 值，亦或使用 $_ENV['HOME'] 来代替 $HOME 获取环境变量 HOME 的值。
从 PHP 4.1.0 开始，PHP 提供了一套附加的预定数组，这些数组变量包含了来自 web 服务器（如果可用），运行环境，和用户输入的数据。这些数组非常特别，它们在全局范围内自动生效，例如，在任何范围内自动生效。因此通常被称为自动全局变量（autoglobals）或者超全局变量（superglobals）。（PHP 中没有用户自定义超全局变量的机制。）超全局变量罗列于下文中；而且，你也将注意到旧的预定义数组（ $HTTP_*_VARS）仍旧存在。自 PHP 5.0.0 起，长格式的 PHP 预定义变量可以通过设置 register_long_arrays 来屏蔽。
下表是PHP的超全局变量：
| 超全局变量 | 描述 |
| $GLOBALS | 包含一个引用指向每个当前脚本的全局范围内有效的变量。该数组的键名为全局变量的名称。从 PHP 3 开始存在 $GLOBALS 数组。 |
| $_SERVER | 变量由 web 服务器设定或者直接与当前脚本的执行环境相关联。类似于旧数组 $HTTP_SERVER_VARS 数组（依然有效，但反对使用）。 |
| $_GET | 经由 URL 请求提交至脚本的变量。类似于旧数组 $HTTP_GET_VARS 数组（依然有效，但反对使用）。 |
| $_POST | 经由 HTTP POST 方法提交至脚本的变量。类似于旧数组 $HTTP_POST_VARS 数组（依然有效，但反对使用）。 |
| $_COOKIE | 经由 HTTP Cookies 方法提交至脚本的变量。类似于旧数组 $HTTP_COOKIE_VARS 数组（依然有效，但反对使用）。 |
| $_FILES | 经由 HTTP POST 文件上传而提交至脚本的变量。类似于旧数组 $HTTP_POST_FILES 数组（依然有效，但反对使用） |
| $_ENV | 执行环境提交至脚本的变量。类似于旧数组 $HTTP_ENV_VARS 数组（依然有效，但反对使用）。 |
| $_REQUEST | 经由 GET，POST 和 COOKIE 机制提交至脚本的变量，因此该数组并不值得信任。所有包含在该数组中的变量的存在与否以及变量的顺序均按照 php.ini 中的 variables_order 配置指示来定义。此数组在 PHP 4.1.0 之前没有直接对应的版本。参见 import_request_variables()。 |
| $_SESSION | 当前注册给脚本会话的变量。类似于旧数组 $HTTP_SESSION_VARS 数组（依然有效，但反对使用) |
每个变量都有应用范围，那么PHP是怎么定义的呢？还是先看看下面代码吧：
1 php
2 $var = 0 ;
3 function test( $index )
4 {
5 $var = $var + 1 ;
6 echo " The " . $index . " number is " . $var . "
" ;
7 }
8 test( 1 );
9 test( 2 )
10 ?>
如果你认为是下面：
结果1：
The 1 number is 1
The 2 number is 2
其实正确的结果应该是：
结果2
The 1 number is 1
The 2 number is 1
1 php
2 $var = 0 ;
3 function test( $index )
4 {
5 global $var ;
6 $var = $var + 1 ;
7 echo " The " . $index . " number is " . $var . "
" ;
8 }
9 test( 1 );
10 test( 2 )
11 ?>
那么还有没有其他方法呢？答案是肯定的。
代码如下：
1 php
2 $var = 0 ;
3 function test( $index )
4 {
5
6 $GLOBALS [ " var " ] = $GLOBALS [ " var " ] + 1 ;
7 echo " The " . $index . " number is " . $GLOBALS [ " var " ] . "
" ;
8 }
9 test( 1 );
10 test( 2 )
11 ?>
PHP也有静态变量的说法。不过静态变量一般用在函数里，只能是局部变量了。看看下面代码吧：
1 php
2 function Test()
3 {
4 static $a = 0 ;
5 echo $a . "
" ;
6 $a ++ ;
7 }
8 Test();
9 Test();
10 ?>
1
2
PHP还有一个相当令人兴奋的特性: 可变变量
所谓可变变量，就是一个变量的变量名可以动态的设置和使用。
看看下面的例子：
1 php
2 $a = " hello " ;
3 $hello = " world " ;
4 echo $a . " " . $ $a ;
5 ?>
变量就这多了。下面看看常量。
常量
PHP的常量是不是前面加const呢？让我们看一看。
不是的。在PHP必须用下面的方式定义。
bool define ( string name, mixed value [, bool case_insensitive] )
name 为常量名，value为常量的值。case_insensitive]为大小写敏感。默认为敏感。例如：
1 php
2 define ( " CONSTANT " , " Hello world. " );
3 echo CONSTANT ; // outputs "Hello world."
4 echo Constant ; // outputs "Constant" and issues a notice.
5
6 define ( " GREETING " , " Hello you. " , true );
7 echo GREETING; // outputs "Hello you."
8 echo Greeting; // outputs "Hello you."
9
10 ?>
常量和变量不同：
常量前面没有美元符号（$）；
常量只能用 define() 函数定义，而不能通过赋值语句；
常量可以不用理会变量范围的规则而在任何地方定义和访问；
常量一旦定义就不能被重新定义或者取消定义；
常量的值只能是标量。
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