Learn how to use eight commonly used magic constants in PHP (detailed examples)

In the previous article "Take you to understand the definition and value acquisition of PHP constants", we introduced the basic knowledge of constants in PHP in detail. In this article, let's take a look at PHP. The knowledge about magic constants in , I hope it will be helpful to everyone!

In the previous article, we learned about the basic knowledge of simple definition and acquisition of constants in PHP, as well as some magic methods. Next, let’s take a look at the relevant knowledge of magic constants, what they are, and what are the main uses of magic constants.

What are magic constants

Magic constants are not exactly constants. We introduced constants in our previous article Note that a constant cannot be changed after it is defined. But magic constants are some constants predefined by PHP. These predefined constants can change due to changes in their location.

Many predefined constants are provided by PHP in any script. Different constants have different extension libraries. Magic constants can be understood as special predefined constants.

The writing format of magic constants is usually two underscores (__) starting with two underscores and ending with two underscores, such as: __LINE__, __FILE__ etc. When it comes to writing format, you need to know that magic constants are not case-sensitive, but try to use uppercase when writing.

Use of magic constants

The PHP operating environment provides eight magic constants, as mentioned above, although they are called Constants, but not exactly constants, their values ​​change with changes in the code. Next, let's take a look at these eight magic variables respectively.

1.<strong><span style="font-size: 16px;">__LINE__ </span></strong> represents the file The current line number in

Next, let’s look at the use of __LINE__ through examples, and look at the value of the magic constant as the position changes. What a change.

The example is as follows:

<?php
echo &#39;这是第 " &#39; . __LINE__ . &#39; " 行<br/>&#39;;
echo &#39;这是第 " &#39; . __LINE__ . &#39; " 行<br/>&#39;;
echo &#39;---------------<br/>&#39;;
echo &#39;这是第 " &#39; . __LINE__ . &#39; " 行&#39;;
?>

Output result:

##In the above example, we use the magic variable __LINE__ Able to get the number of lines at which the current code is located.

What needs to be noted is: We can find that when the location of the code is different, the output results are also different. So we said above that magic constants are not considered constants, and their values ​​change as the code changes. Let’s look down:

2.__FILE__ Represents the absolute path of the file

through

__FILE__The absolute path of the file can be obtained, and this path contains the file name. It should be noted that if the magic constant is used in an included file, the returned result is the name of the included file.

Let’s take a look at the use of __FILE__ through an example. The example is as follows:

<?php
echo &#39;该文件位于 " &#39; . __FILE__ . &#39; " &#39;;
?>

Output result:

In the above example, we used __FILE__ to obtain the absolute path of the file.

3.__DIR__ Used to indicate the directory where the file is located

pass

__DIR__ Can get the directory where the file is located. The directory where the file is located is still different from the absolute path of __FILE__ to obtain the file. The directory where the file is located does not have the file name of the current file. This magic constant can also be understood as dirname(__FILE__).

Let’s take a look at the use of __DIR__ magic constant through an example. The example is as follows:

<?php
echo &#39;该文件位于 " &#39; . __DIR__ . &#39; " &#39;;
echo  dirname(__FILE__);
?>

Output result:

In the above example, we obtained the directory where the current file is located through two methods, one is through the __DIR__ magic constant, and the other is dirname(__FILE__). The effect they can achieve is the same.

4.__FUNCTION__ Used to get the current function name or method name

Through

__FUNCTION__ You can get the name of the current function or method. Next, let’s take a look at the usage of __FUNCTION__ magic constant through an example.

The example is as follows:

<?php
function test() {
 echo &#39;通过__FUNCTION__获取的函数名为：&#39; . __FUNCTION__ ;
}
test();
?>

Output result:

##Through the above example we understand that __FUNCTION__ can obtain the current function Function name.

5.__CLASS__ 用于获取当前的类名

通过__CLASS__可以用于获取当前的类名，齐总也包括了当前类的作用区域或命名空间。接下来我们通过示例来看一下__CLASS__的使用。

示例如下：

<?php
class test {
 function _print() {
  echo &#39;通过__CLASS__获取的类名为：&#39; . __CLASS__ . "<br>";
  echo &#39;通过__FUNCTION__获取的函数名为：&#39; . __FUNCTION__ ;
 }
}
$t = new test();
$t->_print();
?>

输出结果：

通过上述示例已经了解在PHP中可以通过__CLASS__来获取当前的类名。

6.__TRAIT__ 当前的trait名称

通过__TEAIT__能够表示当前的trait名称，这其中包括了trait的作用区域或者命名空间。接下来我们通过示例来看一下__TRAIT__的使用。

示例如下：

<?php
class Base {
 public function sayHello() {
  echo &#39;Hello &#39;;
 }
}
trait SayWorld {
 public function sayHello() {
  parent::sayHello();
  echo &#39;World!&#39;;
 }
}
class MyHelloWorld extends Base {
 use SayWorld;
}
$o = new MyHelloWorld();
$o->sayHello();
?>

输出结果：

7.__METHOD__ 当前的方法名

通过__METHOD__能够获取当前的方法名，其中也包括类名，接下来我们通过示例来看一下__METHOD__的使用方法，示例如下：

<?php
function testMethod()
{
    echo &#39;当前的函数名为:&#39; . __METHOD__ . PHP_EOL . &#39;<br/>&#39;;
}
class TestMethodClass
{
    function testMethod1()
    {
        echo &#39;当前的方法名和类名为:&#39; . __METHOD__ . PHP_EOL; 
    }
}
testMethod();
$test = new TestMethodClass();
$test->testMethod1();
?>

输出结果：

上述示例中，我们通过__METHOD__获取了函数名和方法名。其中方法名中包括了类名。

8.__NAMESPACE__ 当前文件的命名空间名称

通过__NAMESPACE__可以获取当前文件的命名空间名称，接下来我们通过示例来看一下，示例如下：

<?php
namespace MyProject;
echo &#39;命名空间为："&#39;, __NAMESPACE__, &#39;"&#39;; 
?>

输出结果：

到此我们便是通过示例将八种魔术常量了解了一下。

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