Can't Get AES_ENCRYPT to work properly

[Jeremy Freedman]

I an trying to save credit card numbers using AES functions. When I tried a simple test it worked ok like SELECT AES_DECRYPT( AES_ENCRYPT('MEET' , $key) , $key) AS H , it gave H the initial value of 'MEET'

But when tried to use the same thing for the 16 digit Credit card field, it only stored junk like '!V' in the field then returned the same junk when I ran it through the decrypt on the SELECT end of it.
FYI
$key = "'124'"; using PHP
The field is varchar(128)