这里的代码输出20,20，为什么可以在这里访问私有属性：
class myClass {
private $a;
public function __construct() {
$this->a = 10;
}
public function printValue() {
print "The Value is: {$this->a}\n";
}
public function changeValue($val, $obj = null) {
if(is_null($obj)) {
$this->a = $val;
} else {
$obj->a = $val; //why this works?
}
}
public function getValue() {
return $this->a;
}
}
$obj_one = new myClass();
$obj_two = new myClass();
$obj_one->changeValue(20, $obj_two);
$obj_two->changeValue($obj_two->getValue(), $obj_one);
$obj_two->printValue();
$obj_one->printValue();有任何想法吗？
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