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, either dynamically loaded or included at compile time.
There are eight magic constants whose values change depending on their location 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:
The current line number in the file.
<?php echo ' This is line number " ' . __LINE__ . ' " ; ? >
The output result of the above example is:
This is line "2"
The full path and filename 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 ' The file is located at " ' . __FILE__ . ' " ' ; ?>
The output result of the above example is:
The file is located at "E:wampwwwtestindex.php"
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 ' The file is located at " ' . __DIR__ . ' " ' ; ?>
The output result of the above example is:
The file is located at " E:wampwwwtest "
Function name (new 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 this value is always lowercase.
<?php function test ( ) { echo ' Function name: ' . __FUNCTION__ ; } test ( ) ; ?>
The output result of the above example is:
Function name: test
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 this value is always lowercase. The class name includes the scope in which it is declared (eg FooBar). 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 ' Class name: ' . __CLASS__ . " <br> " ; echo ' Function name: ' . __FUNCTION__ ; } } $t = new test ( ) ; $t -> _print ( ) ; ?>
The output result of the above example is:
Class name: test Function name: _print
The name of the trait (new in PHP 5.4.0). Since PHP 5.4.0, PHP has implemented a method of code reuse called traits.
The trait name includes the scope in which it is declared (for example, FooBar).
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!
The method name of the class (new in PHP 5.0.0). Returns the name of the method as it was defined (case-sensitive).
Example:
<?php function test ( ) { echo ' Function name: ' . __METHOD__ ; } test ( ) ; ?>
The output result of the above example is:
Function name: test
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"