PHP provides a large number of predefined constants to any script it runs.

However, many constants are defined by different extension libraries and will only appear when these extension libraries are loaded, or after dynamic loading, or have been included during compilation.

There are eight magic constants whose values ??change depending on their position in the code.

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

__LINE__	The current line number in the file.
# __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 if it is a symbolic link), while versions before that sometimes contained a relative path.
# __DIR__	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)
# __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.
## __CLASS__ ##The name of the class (PHP 4.3. 0 new additions). Since PHP 5 this constant returns the name of the class when it was defined (case sensitive)

__TRAIT__	The name of the Trait (newly added in PHP 5.4.0). Since PHP 5.4.0, PHP implements a method of code reuse called traits.
__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).
__NAMESPACE__	The name of the current namespace (case-sensitive). This constant is defined at compile time (new in PHP 5.3.0).

Let’s explain the current line number in the

---

__LINE__

file one by one.

Example

<?php
 header("Content-type:text/html;charset=utf-8");    //設(shè)置編碼
 echo '這是第 “ '  . __LINE__ . ' ” 行';
 
 ?>

Program running result:

This is Line "3"

---

__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 if it is a symbolic link), while versions before that sometimes contained a

relative paths.

Example

<?php
 header("Content-type:text/html;charset=utf-8");    //設(shè)置編碼
 echo '該文件位于 “ '  . __FILE__ . ' ” ';
 ?>

Program running result:

The file is located in "D:\WWW \11.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)

Example

<?php
 header("Content-type:text/html;charset=utf-8");    //設(shè)置編碼
 echo '該文件位于 “ '  . __DIR__ . ' ” ';
 ?>

Program running result:

This file Located at "D:\WWW"

---

__FUNCTION__

Function name (new 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.

Example

<?php
header("Content-type:text/html;charset=utf-8");    //設(shè)置編碼
function funtext() {
    echo  '函數(shù)名為：' . __FUNCTION__ ;
}
funtext();
?>

Program running result:

The function name is: funtext

---

__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.

Example

<?php
 header("Content-type:text/html;charset=utf-8");    //設(shè)置編碼
 class classtest {
     function fun() {
         echo '類名為：'  . __CLASS__ . "<br>";
         echo  '函數(shù)名為：' . __FUNCTION__ ;
     }
 }
 $t = new classtest();
 $t->fun();
 ?>

Program running result:

Class name For: classtest

The function name is: fun

---

__TRAIT__

The name of Trait (newly added 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.

Example

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

Program running result:

Hello World!

---

__METHOD__

##Method name of the class (PHP 5.0.0 Added). Returns the name of the method as it was defined (case-sensitive).

<?php
 header("Content-type:text/html;charset=utf-8");    //設(shè)置編碼
 function test() {
     echo  '函數(shù)名為：' . __METHOD__ ;
 }
 test();
 ?>

Program running result:

Function name: 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;
 header("Content-type:text/html;charset=utf-8");    //設(shè)置編碼
 echo '命名空間為："', __NAMESPACE__, '"'; // 輸出 "MyProject"
 ?>

Program running result:

The namespace is: "MyProject"