魔术常数是php中的预定义常量，根据它们的使用而改变。 它们以双下划线(__)开头，以双下划线结尾。

它们类似于其他预定义的常量，但是它们随着上下文的改变而改变它们的值，它们被称为魔术常量。

下表中定义了八个魔法常量。 它们不区分大小写。

实例

下面来看看一个上面的每个魔法常量的例子。

文件名：magic.php

<?php

    echo "<h3>example for __line__</h3>";  
    echo "you are at line number " . __line__ . "<br><br>";// print your current line number i.e;3  
    echo "<h3>example for __file__</h3>";  
    echo __file__ . "<br><br>";//print full path of file with .php extension  
    echo "<h3>example for __dir__</h3>";  
    echo __dir__ . "<br><br>";//print full path of directory where script will be placed  
    echo dirname(__file__) . "<br><br>"; //its output is equivalent to above one.  
    echo "<h3>example for __function__</h3>";  
    //using magic constant inside function.  
    function cash(){  
        echo 'the function name is '. __function__ . "<br><br>";//the function name is cash.  
    }  

    cash();  

    //using magic constant outside function gives the blank output.  
    function test_function(){  
        echo 'hyiiii';  
    }  

    test_function();  
    echo  __function__ . "<br><br>";//gives the blank output.  

    echo "<h3>example for __class__</h3>";  
    class abc  
    {  
        public function __construct() {  
            ;  
        }  

        function abc_method(){  
        echo __class__ . "<br><br>";//print name of the class abc.  
        }  
    }  
    $t = new abc;  
    $t->abc_method(); 

    class first{  
        function test_first(){  
            echo __class__;//will always print parent class which is first here.  
        }  
    }  

    class second extends first  
    {  
        public function __construct() {  
            ;  
        }  
    }  
    $t = new second;  
    $t->test_first();  

    echo "<h3>example for __trait__</h3>";  
    trait created_trait{  
        function abc(){  
            echo __trait__;//will print name of the trait created_trait  
        }  
    }  

    class anew{  
        use created_trait;  
    }  
    $a = new anew;  
    $a->abc();  
    echo "<h3>example for __method__</h3>";  

    class meth{  
        public function __construct() {  
            echo __method__ . "<br><br>";//print meth::__construct  
        }  
        public function meth_fun(){  
            echo __method__;//print meth::meth_fun  
        }  
    }  
    $a = new meth;  
    $a->meth_fun();  

    echo "<h3>example for __namespace__</h3>";  
    class name{  
        public function __construct() {  
            echo 'this line will be printed on calling namespace';  
        }  
    }  
    $clas_name= __namespace__ .'\name';  
    $a = new $clas_name;  

?>

执行上面代码得到以下结果 -