PHP 向它运行的任何脚本提供了大量的预定义常量。不过很多常量都是由不同的扩展库定义的，只有在加载了这些扩展库时才会出现，或者动态加载后，或者在编译时已经包括进去了。
有八个魔术常量它们的值随着它们在代码中的位置改变而改变。例如 __LINE__ 的值就依赖于它在脚本中所处的行来决定。这些特殊的常量不区分大小写，如下：
| 名称 | 说明 |
|---|---|
__LINE__ | 文件中的当前行号。 |
__FILE__ | 文件的完整路径和文件名。如果用在被包含文件中，则返回被包含的文件名。自 PHP 4.0.2 起，__FILE__ 总是包含一个绝对路径（如果是符号连接，则是解析后的绝对路径），而在此之前的版本有时会包含一个相对路径。 |
__DIR__ | 文件所在的目录。如果用在被包括文件中，则返回被包括的文件所在的目录。它等价于 dirname(__FILE__)。除非是根目录，否则目录中名不包括末尾的斜杠。（PHP 5.3.0中新增） = |
__FUNCTION__ | 函数名称（PHP 4.3.0 新加）。自 PHP 5 起本常量返回该函数被定义时的名字（区分大小写）。在 PHP 4 中该值总是小写字母的。 |
__CLASS__ | 类的名称（PHP 4.3.0 新加）。自 PHP 5 起本常量返回该类被定义时的名字（区分大小写）。在 PHP 4 中该值总是小写字母的。类名包括其被声明的作用区域（例如 Foo\Bar）。注意自 PHP 5.4 起 __CLASS__ 对 trait 也起作用。当用在 trait 方法中时，__CLASS__ 是调用 trait 方法的类的名字。 |
__TRAIT__ | Trait 的名字（PHP 5.4.0 新加）。自 PHP 5.4 起此常量返回 trait 被定义时的名字（区分大小写）。Trait 名包括其被声明的作用区域（例如 Foo\Bar）。 |
__METHOD__ | 类的方法名（PHP 5.0.0 新加）。返回该方法被定义时的名字（区分大小写）。 |
__NAMESPACE__ | 当前命名空间的名称（区分大小写）。此常量是在编译时定义的（PHP 5.3.0 新增）。 |
参见 get_class()，get_object_vars()，file_exists() 和 function_exists()。
the difference between
__FUNCTION__ and __METHOD__ as in PHP 5.0.4 is that
__FUNCTION__ returns only the name of the function
while as __METHOD__ returns the name of the class alongwith the name of the function
class trick
{
function doit()
{
echo __FUNCTION__;
}
function doitagain()
{
echo __METHOD__;
}
}
$obj=new trick();
$obj->doit();
output will be ---- doit
$obj->doitagain();
output will be ----- trick::doitagainThe __CLASS__ magic constant nicely complements the get_class() function.
Sometimes you need to know both:
- name of the inherited class
- name of the class actually executed
Here's an example that shows the possible solution:
<?php
class base_class
{
function say_a()
{
echo "'a' - said the " . __CLASS__ . "<br/>";
}
function say_b()
{
echo "'b' - said the " . get_class($this) . "<br/>";
}
}
class derived_class extends base_class
{
function say_a()
{
parent::say_a();
echo "'a' - said the " . __CLASS__ . "<br/>";
}
function say_b()
{
parent::say_b();
echo "'b' - said the " . get_class($this) . "<br/>";
}
}
$obj_b = new derived_class();
$obj_b->say_a();
echo "<br/>";
$obj_b->say_b();
?>
The output should look roughly like this:
'a' - said the base_class
'a' - said the derived_class
'b' - said the derived_class
'b' - said the derived_classThere is no way to implement a backwards compatible __DIR__ in versions prior to 5.3.0.
The only thing that you can do is to perform a recursive search and replace to dirname(__FILE__):
find . -type f -print0 | xargs -0 sed -i 's/__DIR__/dirname(__FILE__)/'A lot of notes here concern defining the __DIR__ magic constant for PHP versions not supporting the feature. Of course you can define this magic constant for PHP versions not yet having this constant, but it will defeat its purpose as soon as you are using the constant in an included file, which may be in a different directory then the file defining the __DIR__ constant. As such, the constant has lost its *magic*, and would be rather useless unless you assure yourself to have all of your includes in the same directory.
Concluding: eye catchup at gmail dot com's note regarding whether you can or cannot define magic constants is valid, but stating that defining __DIR__ is not useless, is not!Since namespace were introduced, it would be nice to have a magic constant or function (like get_class()) which would return the class name without the namespaces.
On windows I used basename(__CLASS__). (LOL)What eyecatchup and I posted are good one-liners, but they are fundamentally flawed. Magic constants cannot be defined in a backward-compatible manner in PHP code because the value of a magic constant is defined at run-time based on the current context.
__DIR__ is defined relative to the current file. If you define it in one file using the method that I or eyecatchup posted, then the value is dependant upon the location of the file where it is defined.
Example:
* directory structure:
/dir/other/other_dir.php
/dir/define_dir.php
/dir/same_dir.php
/parent_dir.php
* /dir/define_dir.php
<?php
/**
* Ensure the __DIR__ constant is defined for PHP 4.0.6 and newer.
*/
(@__DIR__ == '__DIR__') && define('__DIR__', realpath(dirname(__FILE__)));
?>
* /dir/same_dir.php
<?php
require('define_dir.php');
echo 'function: ' . realpath(dirname(__FILE__)) . "\n";
echo ' __DIR__: ' . __DIR__ . "\n";
?>
* /dir/other/other_dir.php
<?php
require('../define_dir.php');
echo 'function: ' . realpath(dirname(__FILE__)) . "\n";
echo ' __DIR__: ' . __DIR__ . "\n";
?>
* /parent_dir.php
<?php
require('dir/define_dir.php');
echo 'function: ' . realpath(dirname(__FILE__)) . "\n";
echo ' __DIR__: ' . __DIR__ . "\n";
?>
*** OUTPUT: PHP 5.2.17 ***
same_dir.php:
function: /dir
__DIR__: /dir
other_dir.php:
function: /dir/other
__DIR__: /dir
parent_dir.php:
function: /
__DIR__: /dir
As you can see, only when the running script is in the same directory as the file that defines __DIR__, will __DIR__ have the correct value.
You could use the following function:
<?php
function abspath($file)
{
return realpath(dirname($file));
}
?>
And call it like so wherever you would use __DIR__:
<?php
abspath(__FILE__);
?>
Magic constants are fickle creatures.Note a small inconsistency when using __CLASS__ and __METHOD__ in traits (stand php 7.0.4): While __CLASS__ is working as advertized and returns dynamically the name of the class the trait is being used in, __METHOD__ will actually prepend the trait name instead of the class name!Just learned an interesting tidbit regarding __FILE__ and the newer __DIR__ with respect to code run from a network share: the constants will return the *share* path when executed from the context of the share.
Examples:
// normal context
// called as "php -f c:\test.php"
__DIR__ === 'c:\';
__FILE__ === 'c:\test.php';
// network share context
// called as "php -f \\computerName\c$\test.php"
__DIR__ === '\\computerName\c$';
__FILE__ === '\\computerName\c$\test.php';
NOTE: realpath('.') always seems to return an actual filesystem path regardless of the execution context.As pointed out by david at thegallagher dot net[1], you can NOT use the defined() function to check if a *magic* constant is defined. Often seen, but will not work:
<?php
if (!defined('__MAGIC_CONSTANT__')) {
// FAIL! even if __MAGIC_CONSTANT__ is defined,
// defined('__MAGIC_CONSTANT__') will ALWAYS return (bool)false.
}
?>
Now, raat1979 at gmail dot com[2] pointed out a solution to check if a magic constant is defined or not (which actually works reliable). Thanks to dynamic typecasting in PHP, if a constant lookup fails PHP interprets the given constant name as string (note that a notice is thrown nonetheless. thus, use "@" to suppress it).
<?php
var_dump(@UNDEFINED_CONSTANT_NAME); //prints: string(23) "UNDEFINED_CONSTANT_NAME"
?>
Meaning we can check for all constants - including magic constants (eg __DIR__) - as follows:
<?php
if (@__DIR__ == '__DIR__'){
// __DIR__ was interpreted as string. thus, (magic) constant __DIR__ is not defined.
}
?>
However, what is wrong in raat1979 at gmail dot com's note[2] is this comment:
> "remember that because they are MAGIC constants defining __DIR__ is completely useless"
In fact, you *can* define magic constants (as long as they haven't been defined before, of course).
Based on all I've read and tested today, here is my code I use to make the `__DIR__` magic constant work with all PHP versions (4.3.1 - 5.5.3):
<?php
// Ensure that PHP's magic constant __DIR__ is defined - no matter of the PHP version.
// If magic __DIR__ constant is not defined, define it.
(@__DIR__ == '__DIR__') && define('__DIR__', dirname(__FILE__));
// All PHP versions (>= 4.3.1) can use the magic __DIR__ constant now..
// Demo (outputs and VLD opcodes) here: http://3v4l.org/bm6e1
?>
[1] http://us2.php.net/manual/en/language.constants.predefined.php#107614
[2] http://us2.php.net/manual/en/language.constants.predefined.php#113130Magic constants can not be tested with defined($name)
<?php
if(!defined(__DIR__)){
//will not work
}
?>
when __DIR__ is not defined and you use it anyway php assumes you meant '__DIR__' and throws a notice.
because of this assumption we can do:
<?php
if(@__DIR__ == '__DIR__'){
echo 'magic __DIR__ constant NOT defined';
//insert this code where needed, remember that because they are MAGIC constants defining __DIR__ is completely useless
}echo{
echo 'magic __DIR__ constant IS defined';
}
?>Further clarification on the __TRAIT__ magic constant.
<?php
trait PeanutButter {
function traitName() {echo __TRAIT__;}
}
trait PeanutButterAndJelly {
use PeanutButter;
}
class Test {
use PeanutButterAndJelly;
}
(new Test)->traitName(); //PeanutButter
?>Further to my previous note, the 'object' element of the array can be used to get the parent object. So changing the get_class_static() function to the following will make the code behave as expected:
<?php
function get_class_static() {
$bt = debug_backtrace();
if (isset($bt[1]['object']))
return get_class($bt[1]['object']);
else
return $bt[1]['class'];
}
?>
HOWEVER, it still fails when being called statically. Changing the last two lines of my previous example to
<?php
foo::printClassName();
bar::printClassName();
?>
...still gives the same problematic result in PHP5, but in this case the 'object' property is not set, so that technique is unavailable.You cannot check if a magic constant is defined. This means there is no point in checking if __DIR__ is defined then defining it. `defined('__DIR__')` always returns false. Defining __DIR__ will silently fail in PHP 5.3+. This could cause compatibility issues if your script includes other scripts.
Here is proof:
<?php
echo (defined('__DIR__') ? '__DIR__ is defined' : '__DIR__ is NOT defined' . PHP_EOL);
echo (defined('__FILE__') ? '__FILE__ is defined' : '__FILE__ is NOT defined' . PHP_EOL);
echo (defined('PHP_VERSION') ? 'PHP_VERSION is defined' : 'PHP_VERSION is NOT defined') . PHP_EOL;
echo 'PHP Version: ' . PHP_VERSION . PHP_EOL;
?>
Output:
__DIR__ is NOT defined
__FILE__ is NOT defined
PHP_VERSION is defined
PHP Version: 5.3.6Note that __FILE__ has a quirk when used inside an eval() call. It will tack on something like "(80) : eval()'d code" (the number may change) on the end of the string at run-time. The workaround is:
$script = php_strip_whitespace('myprogram.php');
$script = str_replace('__FILE__',"preg_replace('@\(.*\(.*$@', '', __FILE__,1)",$script);
eval($script);__DIR__ is actually equivalent to realpath(dirname(__FILE__)).
Here's a modified version of the one-liner eyecatchup at gmail dot com[1] wrote:
<?php
// ensure the __DIR__ constant is defined for PHP 4.0.6 and newer
(@__DIR__ == '__DIR__') && define('__DIR__', realpath(dirname(__FILE__)));
?>
Their version also works on PHP 4.0.6, but doesn't use realpath. __DIR__ is an absolute path to the current file.
[1] http://www.php.net/manual/en/language.constants.predefined.php#113233Note that __CLASS__ contains the class it is called in; in lowercase. So the code:
class A
{
function showclass()
{
echo __CLASS__;
}
}
class B extends A
{
}
$a = new A();
$b = new B();
$a->showclass();
$b->showclass();
A::showclass();
B::showclass();
results in "aaaa";Using __METHOD__ always return class name the function belong to. But in some cases (like to make log output), if class B1, B2, B3 inherit class A and call a method of class A, you may would like to see into log which class among B1, B2, B3 is calling.
For example you would like to see:
B1::commond_method_inside_framework
and not
A::commond_method_inside_framework
because you know that common_method_inside_framework is inside common inherited class A.
To solve this, replace __METHOD__ with get_class($this).'::'.__FUNCTION__
class A
{
function commond_method_inside_framework()
{
echo "This is class " . get_class($this).'::'.__FUNCTION__.' '. __METHOD__.' '."\n";
}
}
class B1 extends A
{
function commond_method_inside_caller()
{
echo "This is class " . get_class($this).'::'.__FUNCTION__.' '. __METHOD__.' '."\n";
}
}
$a = new A();
$a->commond_method_inside_framework();
$b = new B1();
$b->commond_method_inside_framework();
$b->commond_method_inside_caller();
?>
Result will be
This is class A::commond_method_inside_framework A::commond_method_inside_framework
This is class B1::commond_method_inside_framework A::commond_method_inside_framework
This is class B1::commond_method_inside_caller B1::commond_method_inside_caller__CLASS__ has caused me some confusion before. If you extend from a class that calls __CLASS__ in a function, __CLASS__ will refer to the parent class and not the extending class.
<?php
class A
{
function sayClassFromObjectA()
{
echo "<br/>This is class " . __CLASS__;
}
}
class B extends A
{
function sayClassFromObjectB()
{
echo "<br/>This is class " . __CLASS__;
}
}
$b = new B();
//I expect it to output "This is class B".
$b->sayClassFromObjectA(); //Outputs "This is class A"
//I expect it to output "This is class B".
$b->sayClassFromObjectB(); //Outputs "This is class B"
?>