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The define() function defines a constant.
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Constants are much like variables, except for the following differences: 
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A constant's value cannot be changed after it is set 
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Constant names do not need a leading dollar sign ($) 
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Constants can be accessed regardless of scope 
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Constant values can only be strings and numbers 
³£Á¿µÄÖµÖ»ÄÜÊÇ“×Ö·û´®[string]”ºÍ“Êý×Ö[number]”£»
Syntax
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define(name,value,case_insensitive)
 
Parameter
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name
Required. Specifies the name of the constant
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value
Required. Specifies the value of the constant
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case_insensitive
Optional. Specifies whether the constant name should be case-insensitive. If set to TRUE, the constant will be case-insensitive. Default is FALSE (case-sensitive)
¿ÉÑ¡²ÎÊý¡£Ö¸¶¨³£Á¿µÄÃû³ÆÊÇ·ñÊDz»Çø·Ö´óСдµÄ[case-insensitive]¡£Èç¹ûÉèÖÃΪTrue£¬Ôò²»Çø·Ö×Öĸ´óСд£»Èç¹ûÉèÖÃΪFalse£¬ÔòÇø·Ö×Öĸ´óСд¡£Ä¬ÈÏÖµÊÇ£ºFalse
Example 1
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Define a case-sensitive constant:
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<?phpdefine("GREETING","Hello you! How are you today?");echo constant("GREETING");?>
The output of the code above will be:
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Hello you! How are you today?
Example 2
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Define a case-insensitive constant:
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<?phpdefine("GREETING","Hello you! How are you today?",TRUE);echo constant("greeting");?>
The output of the code above will be:
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Hello you! How are you today?
The defined() function checks whether a constant exists.
defined()º¯ÊýµÄ×÷ÓÃÊÇ£º¼ì²éÒ»¸ö³£Á¿ÊÇ·ñ´æÔÚ¡£
Returns TRUE if the constant exists, or FALSE


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