爲什麼我不能將$ _SERVER ['DOCUMENT_ROOT']設置爲屬性？

Question

爲什麼我不能將$_SERVER['DOCUMENT_ROOT']設置爲屬性？ 看到示例代碼

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

public function __construct() 
{ 
//code 
} 

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

    } 
} 

Answer 1

在聲明初始化時不能使用其他變量。試試這個：

class foo 
{ 
private $path; 
private $blah; 

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

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

    } 
} 

順便問一下，你確定私人是合適的選擇，經常被保護是可取的。

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，你需要初始化它們的構造就像mathroc說。