为什么php $ mysqli-> affected_rows在行受影响时返回0？

Question

For my following sql update query I need to get the number of affected rows. With $mysqli->affected_rows, I am getting 0 rows affected but while I check database, data are being updated. I ran the same query from phpMyAdmin which returns the number of affected as expected but my PHP is returning just 0; Any idea where I messed up?

print_r($sql);
// $sql=  UPDATE `f`.`air_pax`  SET `pax_type`='SRC'  , `pax_name`='PAUL/SATTYAJ MSTR(CHD/05NOV04)'  , `fare`='198'  , `c_discount`='1'  , `c_charge`='30'  , `tax`='20'  , `invoice`='247'  , `v_invoice`='ghi'  , `gross_fare`='150'  , `commission`='10'  , `v_charge`='50'  , `apc`='1'  , `t_charge`='5'  , `remarks`='rem 3'  , `cost_per_pass`='212.5'  WHERE `air_pax`.`serial`='5'  LIMIT 1 

if($result = $mysqli->query($sql)){
    var_dump($mysqli->affected_rows); // this returns 'int 0'

    if($mysqli->affected_rows == 1){
        return TRUE;
    } else{
        return FALSE;
    }
}

I know that if no changes are there, then number of affected rows will be 0; My [SERVER_SOFTWARE] => Apache/2.4.9 (Win32) PHP/5.5.12 .