I'm stuck of the requests linking php5 on backports.org against "MySQL5".
You need support of MySQL5 or the mysqli extention for your php5 packages for sarge? So the packages on backports.org are not what you want.
The packages are compiled against libmysqlclient12 (mysql 4.0) cause all apache modules (on backports.org) are compiled against this version. Linking php5 against an other version will lead you in trouble when using these apache modules and php5 together, cause sarge doesn't support "versioned symbols".