PHP study notes_4_Points to note about constants and magic constants

Constants correspond to variables in php. Constants are sub-system constants, framework constants, and custom constants
To define constants, you can use the following

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

Notes:
1. Use it directly without adding the '$' sign;
2. Constants can be defined and used anywhere;
3. You can use the constant name or constant (constant name) function to get the value of the constant;
4.get_defined_constants(); can obtain all defined constants;

Magic constants: added by different extension libraries. Commonly used magic constants are as follows:

Name	Description
__LINE__	The current line number in the file.
__DIR__	The directory where the current file is located.
__FILE__	The full path and file name of the file. If used in an include file, returns the include file name. Since PHP 4.0.2, FILE always contains an absolute path, while versions before that sometimes contained a relative path.
__FUNCTION__	Function name (newly added in php 4.3.0). Since PHP 5 this constant returns the name of the function when it was defined (case sensitive). In php 4 the value is always lower case.
__CLASS__	The name of the class (new in php 4.3.0). Since PHP 5 this constant returns the name of the class when it was defined (case sensitive). In php 4 the value is always lower case.
__METHOD__	The method name of the class (newly added in php 5.0.0). Returns the name of the method when it was defined (case-sensitive).
__NAMESPACE__	The name of the current namespace (case sensitive). This constant is defined at compile time (new in PHP 5.3.0)

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