brew php mcrypt

For Mac users you can install it via Homebrew

brew install mcrypt

and also

brew install homebrew/php/php70-mcrypt

for other versions of php try searching brew first

brew search mcrypt

from : https://laracasts.com/discuss/channels/general-discussion/use-of-undefined-constant-mcrypt-rijndael-128-assumed-mcrypt-rijndael-128?page=1