PHP魔术方法示例

Question

我在Zend PHP学习指南中有这个问题，但找不到合适的解释……

<?php
    class Magic {
        public $a = "A";
        protected $b = array("a"=>"A", "b"=>"B", "c"=>"C");
        protected $c = array(1,2,3);

        public function __get($v) {
            echo "$v,";
            return $this->b[$v];
        }
        public function __set($var, $val) {
            echo "$var: $val,";
            $this->$var = $val;
        }
    }

    $m = new Magic();
    echo $m->a.",".$m->b.",".$m->c.",";
    $m->c = "CC";
    echo $m->a.",".$m->b.",".$m->c;
?>

根据本指南，解决方案应为"b,c,A,B,C,c: CC,b,c,A,B,C“。我不明白为什么--也许你明白？我的意图是第一次调用$m->a会导致结果"a"，但这显然是错误的……

Answer 1

因为__get()调用echo，所以在类外部的echo被调用之前就输出了一些数据。

单步执行echo的第一行，下面是它的执行方式：

$m->a   "A" is concatenated
","     "," is concatenated
$m->b   "b," is echoed, "B" is concatenated
","     "," is concatenated
$m->c   "c," is echoed, "C" is concatenated
"m"     "," is concatenated

此时，已经对b,c,执行了echo操作，并且现在显示了值为A,B,Cm的字符串。