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常量

常量是一个简单值的标识符(名字)。如同其名称所暗示的,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)。常量默认为大小写敏感。通常常量标识符总是大写的。

常量名和其它任何 PHP 标签遵循同样的命名规则。合法的常量名以字母或下划线开始,后面跟着任何字母,数字或下划线。用正则表达式是这样表达的:[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Example #5 合法与非法的常量名

                               <p class="sycode">                           <?         php         //          合法的常量名                  define(         "         FOO         "         ,          "         something         "         );define(         "         FOO2         "         ,          "         something else         "         );define(         "         FOO_BAR         "         ,          "         something more         "         );         //          非法的常量名                  define(         "         2FOO         "         ,          "         something         "         );         //          下面的定义是合法的,但应该避免这样做:(自定义常量不要以__开头)         //          也许将来有一天PHP会定义一个__FOO__的魔术常量         //          这样就会与你的代码相冲突                  define(         "         __FOO__         "         ,          "         something         "         );         ?>                               </p>               


Note: 在这里,字母是 a-z,A-Z,以及从 127 到 255(0x7f-0xff)的 ASCII 字符。

和 superglobals 一样,常量的范围是全局的。不用管作用域就可以在脚本的任何地方访问常量。有关作用得更多信息请阅读手册中的变量范围。

define定义全局常量,在任何地方都可以访问。
const是定义类的全局变量,且不能改变其值。

 

                  <p class="sycode">                 <?     php     class      say_const{       const      CHARSET     =     ”中国”;    publice function say_hello(){       echo slef::CHARSET;    }}  $const1     =     new      say_const()’  $const1     ->     say_hello();     ?>                  </p>


作为其它要说明的,请看下面这个扩展阅读:

扩展阅读: 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; 常量只能包含标量数据(boolean,integer,float 和 string),不要定义 resource常量. 可以用函数 constant() 来读取常量的值.get_defined_constants()可以获得所有已定义的常量列表. 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. PHP 的“魔术常量”.

 

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魔术常量

PHP 向它运行的任何脚本提供了大量的预定义常量。不过很多常量都是由不同的扩展库定义的,只有在加载了这些扩展库时才会出现,或者动态加载后,或者在编译时已经包括进去了。

有七个魔术常量它们的值随着它们在代码中的位置改变而改变。例如 __LINE__ 的值就依赖于它在脚本中所处的行来决定。这些特殊的常量不区分大小写,如下:

 

几个 PHP 的“魔术常量” 名称 说明
__LINE__ 文件中的当前行号。
__FILE__ 文件的完整路径和文件名。如果用在被包含文件中,则返回被包含的文件名。自 PHP 4.0.2 起,__FILE__ 总是包含一个绝对路径(如果是符号连接,则是解析后的绝对路径),而在此之前的版本有时会包含一个相对路径。
__DIR__ 文件所在的目录。如果用在被包括文件中,则返回被包括的文件所在的目录。它等价于 dirname(__FILE__)。除非是根目录,否则目录中名不包括末尾的斜杠。(PHP 5.3.0中新增) =
__FUNCTION__ 函数名称(PHP 4.3.0 新加)。自 PHP 5 起本常量返回该函数被定义时的名字(区分大小写)。在 PHP 4 中该值总是小写字母的。
__CLASS__ 类的名称(PHP 4.3.0 新加)。自 PHP 5 起本常量返回该类被定义时的名字(区分大小写)。在 PHP 4 中该值总是小写字母的。
__METHOD__ 类的方法名(PHP 5.0.0 新加)。返回该方法被定义时的名字(区分大小写)。
__NAMESPACE__ 当前命名空间的名称(大小写敏感)。这个常量是在编译时定义的(PHP 5.3.0 新增)

   echo __FILE__;

  echo __CLASS__;

  echo __file__;
?>

参见 get_class(),get_object_vars(),file_exists() 和 function_exists()。

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