A file's ctime is it's inode change time. The inode changes when file metadata changes (for example when file permissions change). The inode also changes whenever the file's contents change, but since the inode changes for other reasons, it's more accurate to use mtime to get the age of the contents of a file. See SPLFileInfo::getMTime
Also, please note ctime is not creation time. (Most UNIX-like filesystems do not record a file's creation time.)