PHP magic constants
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.
What are the magic constants in PHP?
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 '这是第 “ ' . __LINE__ . ' ” 行'; ?>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.
Example:
<?php echo '该文件位于 “ ' . __FILE__ . ' ” '; ?>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)
Example:
<?php echo '该文件位于 “ ' . __DIR__ . ' ” '; ?>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.
Example:
<?php
function test() {
echo '函数名为：' . __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.
Example:
<?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: 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 'Hello ';
}
}
trait SayWorld {
public function sayHello() {
parent::sayHello();
echo 'World!';
}
}
class MyHelloWorld extends Base {
use SayWorld;
}
$o = new MyHelloWorld();
$o->sayHello();
?> The above routine will output: Hello World!
##__METHOD__ class Method name (new 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: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). Example:
<?php namespace MyProject; echo '命名空间为："', __NAMESPACE__, '"'; // 输出 "MyProject" ?>The output result of the above example is:
命名空间为："MyProject"
