从manual上知道了mysql_affected_rows函数当UPDATE前后的数据一样时会返回异常值， 下面有个方便的解决办法，从官方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.
复制代码 代码如下:
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：因为这个小问题折腾了半天，感觉php真是太不清晰了