ciao ragazzi,ho risolto....devo ringraziare Emulman che mi ha dato la dritta giusta.

Ho seguito il consiglio di andare su http://www.apachelounge.com e li ho trovato questo:

<<Coincidentally, I was just setting up a new system earlier today (Apache 2.2.2, PHP 5.1.4, MySQL 5.0.12). I had a similiar problem - At Apache START the php_mysqli extension was rejected (actually stated that it could not be found - even though it was in the extensions folder). I downloaded the latest extension from MySQL which includes the libMySQL.dll.

http://dev.mysql.com/downloads/connector/php/

Once I copied both the php_mysqli.dll and libMySQL.dll to my extensions folder - PHP loaded the extension. >>

grazie ancora
ciao O