可变的外部范围

Question

的$base在下面的代码变量是显然的范围之外,但是$urlS和$urlE变量都没有？有人可以解释为什么会这样吗？

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

您需要包括self::前$base,像你一样用$urlS和$urlE:

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