define——定义常量,若定义true，则读取时不区分大小写

bool define( string name, mixed value [, bool case_insensitive])

常量只能包含标量数据（boolean ，integer ，float 和 string ）。

define ( "CONSTANT" , "Hello world." );
echo CONSTANT ; // outputs "Hello world."
echo Constant ; // outputs "Constant" and issues a notice.

define ( "GREETING" , "Hello you." , true );
echo GREETING ; // outputs "Hello you."
echo Greeting ; // outputs "Hello you."

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constant()——读取常量

mixed constant ( string name)

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define ( "MAXSIZE" , 100 );

echo MAXSIZE ;
echo constant ( "MAXSIZE" ); // same thing as the previous line

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defined——判断常量是否定义

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bool defined ( string name)

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if ( defined ( 'CONSTANT' )) {
????echo CONSTANT ;
}

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预定义常量

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__FILE__?? 取得文件的物理地址? 注：左右各2条下划线

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echo __FILE__? ?? //C:\wamp\www\T1\1.php

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