How to use php magic constants

Magic constants are a large number of predefined constants that PHP provides to any script it runs. However, many constants are defined by different extension libraries and will only appear when these extension libraries are loaded, either dynamically loaded or included at compile time. Below we will give you a detailed introduction to the usage of various magic constants in PHP.

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##__LINE__

The current line number in the file.

<?php
    echo &#39;这是第 " &#39;  . __LINE__ . &#39; " 行&#39;;
 ?>

The output result of the above example is:

这是第 “ 2 ” 行

---

__FILE__

The full path and file name of the file. If used within an included file, returns the name of the included file.

Since PHP 4.0.2, __FILE__ always contains an absolute path (or the resolved absolute path in the case of a symbolic link), while versions before that sometimes contained a relative path.

<?php
    echo &#39;该文件位于 " &#39;  . __FILE__ . &#39; " &#39;;
?>

The output result of the above example is:

该文件位于 “ E:\wamp\www\test\index.php ”

---

__DIR__

The directory where the file is located. If used within an included file, returns the directory where the included file is located.

It is equivalent to dirname(__FILE__). Directory names do not include the trailing slash unless they are the root directory. (New in PHP 5.3.0)

<?php
    echo &#39;该文件位于 " &#39;  . __DIR__ . &#39; " &#39;;
?>

The output result of the above example is:

该文件位于 “ E:\wamp\www\test ”

---

#__FUNCTION__

Function name (newly added in PHP 4.3.0). Since PHP 5 this constant returns the name of the function as it was defined (case sensitive). In PHP 4 this value is always lowercase.

<?php
    function test() {
        echo  &#39;函数名为：&#39; . __FUNCTION__ ;}test();
?>

The output result of the above example is:

函数名为：test

##__CLASS__

---

The name of the class (newly added 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 this value is always lowercase. The class name includes the scope in which it is declared (e.g. Foo\Bar). Note that since PHP 5.4 __CLASS__ also works for traits. When used within a trait method, __CLASS__ is the name of the class that calls the trait method.

<?php
    class test {
        function _print() {
            echo &#39;类名为：&#39;  . __CLASS__ . "<br>";        
            echo  &#39;函数名为：&#39; . __FUNCTION__ ;    
         }
     }
    $t = new test();
    $t->_print();
 ?>

The output result of the above example is:

类名为：test
函数名为：_print

__TRAIT__

---

## The name of the #Trait (new in PHP 5.4.0). Since PHP 5.4.0, PHP implements a method of code reuse called traits. The Trait name includes the scope in which it is declared (for example, Foo\Bar).

Members inherited from the base class are overridden by the MyHelloWorld method in the inserted SayWorld Trait. Its behavior is consistent with the methods defined in the MyHelloWorld class. The order of precedence is that methods in the current class override trait methods, which in turn override methods in the base class.

<?php
    class Base {
        public function sayHello() {
            echo &#39;Hello &#39;;  
      }
    }
    trait SayWorld {
        public function sayHello() {
            parent::sayHello();        
            echo &#39;World!&#39;;    
           }
       }
     class MyHelloWorld extends Base {
            use SayWorld;
       }
    $o = new MyHelloWorld();
    $o->sayHello();
 ?>

The above routine will output:

Hello World!

__METHOD__

---

#The method name of the class (newly added in PHP 5.0.0). Returns the name of the method as it was defined (case-sensitive).

<?php
    function test() {
        echo  &#39;函数名为：&#39; . __METHOD__ ;
       }
     test();
 ?>

The output result of the above example is:

函数名为：test

##__NAMESPACE__

---

The name of the current namespace (case-sensitive). This constant is defined at compile time (new in PHP 5.3.0).

<?php
    namespace MyProject; 
    echo &#39;命名空间为："&#39;, __NAMESPACE__, &#39;"&#39;; 
 ?>

The output result of the above example is:

命名空间为："MyProject"

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