The function ctype_digit() is used in get_attachment_link() in link-template.php, and I've had at least one user tell me that this broke the upload functionality for him.
He reports running PHP 4.3.10 and getting this error:
[Tue Apr 01 23:32:51 2008] [error] [client x.x.x.x] PHP Fatal
error: Call to undefined function: ctype_digit() in /xxx/wp-includes/link-template.php on line 182,
referer: http://xxx/wp-admin/media-upload.php?type=image&tab=library&post_id=1&post_mime_type=&s=&m=200804
He also reports that replacing it with is_numeric appears to work fine.
Similar ticket: #5481