It would depend on which of the cyphers listed at http://www.php.net/manual/en/mcrypt.ciphers.php that you decide to use - most should at most add a byte or two to the length of whatever you feed into it.
I ask this because I was looking at the CodeIgniter encryption class and their guide states:
Message Length
It’s important for you to know that the encoded messages the encryption function generates will be approximately 2.6 times longer than the original message. For example, if you encrypt the string “my super secret data”, which is 21 characters in length, you’ll end up with an encoded string that is roughly 55 characters (we say “roughly” because the encoded string length increments in 64 bit clusters, so it’s not exactly linear). Keep this information in mind when selecting your data storage mechanism. Cookies, for example, can only hold 4K of information.
I agree…generally encrpyption do increase the text length…plus we have extra work or encryption and decryption(thats why we encrypt important parts only)
ya when i use to use CI,I think i have also read it about nature of its default encrypter.If you dont like it i think you can play with it to get another encrypter.
The standard license of ioncube cost around $400 so,may not be good for small general purpose application.
You need have ioncube installed in your deployment server(which most servers have now a days)
plus it is not unbreakable encryption,now a days there are many people claiming to decrypt it for very negligible fees…
so,it is having wall and leaving other things to ability of cracker…