Devi abilitare l'estensione mbstring nel file php.ini
nel file php.ini trovi una parte più o meno così:
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
devi levare il ";" davanti a 'extension=php_mbstring.dll'
salvare e riavviare apache