为什么我不能将$ _SERVER ['DOCUMENT_ROOT']设置为属性？

Question

为什么我不能将$_SERVER['DOCUMENT_ROOT']设置为属性？ 看到示例代码

class foo 
{ 
private $path = $_SERVER['DOCUMENT_ROOT']; // generates error 
private $blah; 

public function __construct() 
{ 
//code 
} 

    public function setBla($bla) 
    { 
    $this->blah = $bla; 

    } 
} 

Answer 1

在声明初始化时不能使用其他变量。试试这个：

class foo 
{ 
private $path; 
private $blah; 

public function __construct() 
{ 
$this->$path = $_SERVER['DOCUMENT_ROOT']; 
//code 
} 

    public function setBla($bla) 
    { 
    $this->blah = $bla; 

    } 
} 

顺便问一下，你确定私人是合适的选择，经常被保护是可取的。

Answer 2

Class properties只能用常量初始化：

[…] declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

所以，你需要初始化它们的构造就像mathroc说。