Open up your php.ini file, it will be in your the directory you installed php usually c:\Program Files\php\ or c:\php.
Look down to around line 529 for this entry
extension_dir
and take note of what it says if it's
extension_dir = "./ext/"
leave it as it is and look at around line 657 for this
extension=php_mysql.dll
if there is a
;
at the start of the line delete it.
If
extension_dir
is
extension_dir = "./"
you will have to move the dll files in c:\Program Files\php\ext or c:\php\ext into your c:\Program Files\php\ or c:\php\ directory as well as deleting the
;
as described above.
Save the file and restart the Apache server to make the changes take effect.