I dont recommend you use that method, since encryption using PHP's built in functions results in non-sorted values.
By that I mean that it's not necessary for a value bigger than another to have a bigger value as an encrypted string. (see the example in the above post).
I suggest you create a file/sql table and store their names/emails in it.
You can store the md5 value since it doesn't change, i.e., static for the same string.
example :: if you'd like to encrypt 'k' -> 8ce4b16b22b58894aa86c421e8759df3
ofcourse, I dont recommend just encrypting an email, try adding the id to it like this :: $cryptMe=md5($ID.someone@email.com);
If you'd like a secure portal system, go to www.hotscripts.com and search there, they have many free & commercial ones... if you're to sell things online, make you use SSL (https).
Hope that helps.