__isset魔术方法

class Foo {
    public function __isset($varname) {
        echo "ZOMG! isset was called on my $varname!\n";
    }
}
$foo = new Foo;
// bar will never get checked
var_dump(isset($foo->foo, $foo->bar));

// Output:
// ZOMG! isset was called on my foo!
// bool(false)

当用isset函数检测一个对象里的属性是否设置时会自动调用__isset()方法