admin/environment/php extension/ctype

The ctype extension is enabled by default in PHP since version 4.3.0. If it's not enabled on your server, either you have a very old version of php, or it was probably disabled at build time. You will need to either remove --disable-ctype from the build command, or add --enable-ctype. Read http://www.php.net/manual/en/ctype.installation.php for details.

For details about Ctype features and history, you can read http://en.wikipedia.org/wiki/Ctype.h