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.
For example, the value of __LINE__ depends on the line it is located in the script. These special constants are not case-sensitive and are as follows:
__LINE__
The current line number in the file.
Example:
<?php
echo 'This is the line " ' . __LINE__ . ' ";
?>
The output result of the above example is similar to:
This is the "2" line
##__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.
Example:
<?php
echo echo 'The file is located at " ' . __FILE__ . ' " ';
?>
The output result of the above example is similar to:
The file is located at "E:\wamp\www\test\index.php" //This is the absolute path of the file
__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
echo echo 'The file is located at " ' . __DIR__ . ' " ';
?> ;
The output result of the above example is similar to:
The file is located in "E:\wamp\www\test"
__FUNCTION__
The name of the function where the current code is located (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.
Example:
<?php
function test()
{
echo '函数名为：' . __FUNCTION__ ;
}
test();
?>The output result of the above example is:
The function name is: test
__CLASS__
The name of the current 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 '类名为：' . __CLASS__ . "<br>";
echo '函数名为：' . __FUNCTION__ ;
}
}
$t = new test();
$t-> print();
?>The output result of the above example is:
Class name: test
Function name: print
The class (class) will be introduced in detail later in PHP Object-Oriented.
__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 'Hello';
}
}
trait SayWorld
{
public function sayHello()
{
parent::sayHello(); //parent:: 用于调用父类的方法
echo 'World!';
}
}
class MyHelloWorld extends Base
{ //extends 用来继承一个类
use SayWorld;
}
$o = new MyHelloWorld(); //使用 new 运算符来实例化该类的对象
$o->sayHello();
?>The output result of the above example is:
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).
Example:
<?php
function test()
{
echo '函数名为：' . __METHOD__ ;
}
test();
?>The output result of the above example is:
The function name is: test
__NAMESPACE__
The name of the current namespace (case-sensitive). This constant is defined at compile time (new in PHP 5.3.0).
Example:
<?php
namespace MyProject;
echo 'The namespace is: "', __NAMESPACE__, '"'; // Output "MyProject"
?>
The output result of the above example is:
The namespace is: "MyProject"
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