After the Yosemite update yesterday, something is messed up with the Mcrypt extension. The PHP based artistan CLI of the Laravel framework returns:
M
I just had the same problem.
Just use PHP Version 5.5.17 in MAMP and everything should work fine again.
Remember to switch the version in your .bash_profile too:
export PATH=/Applications/MAMP/bin/php/php5.5.17/bin:$PATH