[PHP] Cookie Security?

[phpmail]

My login script sets unique, secure, cookies that identify the user. Some of my pages 
only display content if a secure cookie is present. Is this a bad idea for secure 
pages with sensitive details as I have heard that cookies can be faked? I am always 
interested in creating a secure environment for my website visitors and I want to make 
sure I am protecting their privacy. Any help on this matter is greatly appreciated.
 
Thanks, 
 
~Sean V.