PHP Learning Notes _4_ constants and magic constants points to note

In PHP, variables correspond to constants, constants are system constants, frame constants, and custom constants.
Defining constants can use the following

    
         define("MY_FIELD",123);    echo MY_FIELD;// 输出123// 如果定义常量是一个变量$name = "TEST";    define($name,"456");    echo$name;// 输出TESTecho constant($name);//输出真正的变量值456?>

Note the point:
1. Use directly, without adding ' $ ' sign;
2. Constants can be defined and used anywhere;
3. You can use the constant name or the constant (constant name) function to get the value of the constant;
4.get_defined_constants (); Can get all the constants that have been defined;

Magic constants: added by different extension libraries, the usual magic constants are the following:

name	Description
__LINE__	The current line number in the file.
__DIR__	The directory where the current file resides.
__FILE__	The full path and file name of the file. If used in the include file, the include filename is returned. Since PHP 4.0.2, FILE always contains an absolute path, and the previous version sometimes contains a relative path.
__FUNCTION__	The name of the function (PHP 4.3.0 new addition). From PHP 5 This constant returns the name (case-sensitive) when the function is defined. In PHP 4, this value is always in lowercase letters.
__CLASS__	The name of the class (PHP 4.3.0 new addition). From PHP 5 This constant returns the name of the class when it is defined (case-sensitive). In PHP 4, this value is always in lowercase letters.
__METHOD__	The method name of the class (PHP 5.0.0 new addition). Returns the name of the method when it is defined (case-sensitive).
__NAMESPACE__	The name of the current namespace (case sensitive). This constant is defined at compile time (PHP 5.3.0 new)

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