范围外的变量

Question

下面代码中的$base 变量显然超出了范围，但是$urlS 和$urlE 变量不是吗？谁能解释这是为什么？

class Yahoo {
    //Code below to retrieve live currency rates from yahoo
private static $base = 'USD';
private static $urlS = LIVE_FEED_YAHOO;
private static $urlE = '=X&f=l1';
private $rate;

public function __construct($toCurr) {
$url = self::$urlS.$base.$toCurr.self::$urlE;

Answer 1

您需要在$base 之前包含self::，就像您对$urlS 和$urlE 所做的那样：

$url = self::$urlS . self::$base . $toCurr . self::$urlE;