Nel file php.ini c'é una sezione che é più o meno così:

;Windows Extensions
;Note that ODBC support is built in, so no dll is needed for it.
;

extension=php_pdo.dll
extension=php_bz2.dll
extension=php_curl.dll
extension=php_dba.dll
extension=php_dbase.dll
;extension=php_exif.dll
extension=php_fdf.dll
;extension=php_filepro.dll
extension=php_gd2.dll
extension=php_gettext.dll
;extension=php_ifx.dll
;extension=php_iisfunc.dll
;extension=php_imap.dll
;extension=php_interbase.dll
;extension=php_ldap.dll
extension=php_mbstring.dll
extension=php_mcrypt.dll
extension=php_mhash.dll
extension=php_mime_magic.dll
;extension=php_ming.dll
;extension=php_mssql.dll
;extension=php_msql.dll
extension=php_mysql.dll
;extension=php_oci8.dll
;extension=php_openssl.dll
extension=php_pgsql.dll
;extension=php_shmop.dll
;extension=php_snmp.dll
extension=php_sockets.dll
;extension=php_sybase_ct.dll
;extension=php_tidy.dll
extension=php_xmlrpc.dll
extension=php_xsl.dll
extension=php_sqlite.dll

se hai un ; prima di extension=php_mysql.dll devi levarlo, salvare, e riavviare il webserver