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mixed constant(string constant_name)
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bool defained(string constants_name)
constant_nameΪҪ»ñÈ¡³£Á¿µÄÃû³Æ£¬´æÔÚÔò·µ»Øtrue£¬·ñÔò·µ»Øfalse;
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  define ("MESSAGE", "PHP³£Á¿¶¨Òå,³£Á¿ÃûÇø±ð´óСд");
  echo MESSAGE."<br/>";    //Êä³ö³£Á¿MESSAGE
  echo Message."<br/>";    //Êä³ö"Message"£¬±íʾûÓиó£Á¿
  
  define("MESSAGE2", "PHP³£Á¿¶¨Òå,³£Á¿Ãû²»Çø±ð´óСд", true);
  echo MESSAGE2."<br/>";   //Êä³ö³£Á¿MESSAGE2
  echo Message2."<br/>";   //Êä³ö³£Á¿
  
  $constant_name = "message2";
  echo constant($constant_name)."<br/>";   //Êä³ö³£Á¿MESSAGE2
  echo defined("MESSAGE")."<br/>";         //Èç¹û¶¨Òå·µ»Øtrue,echoÊä³öÏÔʾ1
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Php Variable Circular References

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