affected_rows property is kinda readonly property, so we can't change
that with reflection apis or property value update. 

it makes the property reliable, I don't see a reason to change that,
I you really need to do that inherit it with a subclass.

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

mysqli link properties are designed to be read-only.

the write func for them throw that error:

static int mysqli_write_na(mysqli_object *obj, zval *newval TSRMLS_DC)
{
    php_error_docref(NULL TSRMLS_CC, E_ERROR, "Cannot write property");
    return FAILURE;
}

I understand that making the properties (not only affected_rows) readonly 
ensures certain integrity. No arguing there.

What I'm trying to do is mock the MySQLi class (and some others) for unit 
testing. Mocking the methods works fine, but the properties have been a bit of a 
difficulty. My solution so far was to hook in behind MySQLi and mock the actual 
database connection using mysqlnd_uh. But that is not working (yet) with 5.4 and 
no real estimations of when that will be fixed. With 5.5 drawing nearer, I was 
looking at alternative ways to do this. I tried subclassing MySQLi as well, that 
didn't work either.

hmm, yes, subclass still use that write_func.

a solution might be use composite, like:

class MysqliMock {
   public $mysqli;
   public function __construct() {
        $this->mysqli = new MySQLi();
   }
   public function __call($method, $args) {
        call_user_func_array(array($this->mysqli, "method"), $args)
       
   }

   public function __get($name) {
     //you can mock the properties here
   }

   public function __set($name, $value) {

   }
}
?>

what do you think? thanks

Thanks for the hint. That indeed works like a charm :)