從manual上知道了mysql_affected_rows函數當UPDATE前后的數據一樣時會返回異常值，
下面有個方便的解決辦法，從官方munual上看到 bdobrica at gmail dot com 留言的：
As a solution to the problem pointed in the post reffering to mysql_affected_rows() returning 0 when you are making an update query and the fields are not modified although the query is valid, i'm posting the following function. It is very simple and based on a previous post.
function mysql_modified_rows () {
$info_str = mysql_info();
$a_rows = mysql_affected_rows();
ereg("Rows matched: ([0-9]*)", $info_str, $r_matched);
return ($a_rows < 1)?($r_matched[1]?$r_matched[1]:0):$a_rows;
}
PS：因為這個小問題折騰了半天，感覺php真是太不清晰了
