After upgrading or installing WordPress 2.5, the following PHP error may appear on web pages or in Apache web server log, especially when using the new Flash based media uploader to upload photos, pictures, images, videos and other files:
[error] [client x.x.x.x] PHP Fatal Error: Call to undefined function: ctype_digit() in /wordpress/wp-includes/link-template.php on line 182, referer: /wp-admin/media-upload.php?type=image&tab=library&post_id=1&post_mime_type=&s=&m=200804
The function ctype_digit() is used in get_attachment_link() in link-template.php, and the error may happens on some PHP installation regardless of version. PHP 4 (such 4.3.10) and PHP 5 (such as 5.2.6) both may be affected with the error.
This is a known bug that will be fixed in WordPress 2.5.1, which may released in early May. Meanwhile, if you want to fix the issue immediately, download fixed link-template.php file, and then upload and replace/overwrite the same link-template.php file in /wp-includes/ folder on the web server.