php_mbstring.dll

Hi there,
I have the MSAS 4.5.2 version installed on my local machine. I need to enable the mbstring extension in the php.ini.
I went to /usr/local/Apache2/php.ini to enable the extension by uncommenting the line ;extension=php_mbstring.dll
This should have automatically created the php_mbstring.dll library in the usr/local/PHP/extensions/ directory - right???
But it does not.
When i restart Apache it says unknown() php_mbstring.dll could not be found...
Is there a way to overcome this issue?
By the way, is there a way to upgrade to PHP5 and still keep the rest of the MSAS configuration (I mean Mysql and phpmyadmin versions) same???
I am hearing that PHP5 is much better equipped than php4.
I really need help with this php_mbstring.dll extension. I even tried downloading the binary from the php site. Now i have the php_mbstring.dll extension in my usr/local/PHP/extension/ directory but the error still pops up saying that it could not find the dll file in the extensions directory.
Please get back to me ASAP.
thanks
Hippocket