php__set(）重载了方法的意外行为

我有：

class Address {
    private $number;
    private $street;
    public function __construct( $maybenumber, $maybestreet = null ) {
        if( is_null( $maybestreet) ) {
            $this->streetaddress = $maybenumber;
        } else {
            $this->number = $maybenumber;
            $this->street = $maybestreet;
        }
    }
    public function __set( $property, $value ) {
        if( $property === "streetaddress" ) {
            if( preg_match( "/^('d+.*?)['s,]+(.+)$/", $value, $matches ) ) {
                $this->number = $matches[1];
                $this->street = $matches[2];
            } else {
                throw new Exception( "unable to parse street address: '{$value}'" );
            }
        }
    }
    public function __get( $property ) {
        if( $property === "streetaddress" ) {
            return $this->number . " " . $this->street;
        }
    }
}
$address = new Address( "441b Bakers Street" );
echo "<pre>";
print_r($GLOBALS);
echo "</pre>";

输出：

 ...
[address] => Address Object
        (
            [number:Address:private] => 441b
            [street:Address:private] => Bakers Street
        )

当__set方法甚至没有从任何地方调用时，__set方法是如何被调用的，属性$number和$street是如何设置的？

我的正常逻辑告诉我，当实例化发生时，所有发生的事情都是创建属性streetaddress，并将值传递给$maybenumber参数，因为第二个参数$maybestreet为null。

对这种行为的任何解释都会很有帮助，官方文件的链接也会很好。