Good Encryption

Hey, I have a few of questions that I haven't been able to find easily elsewhere, at least for a lay person ;) If the key size for TWOFISH_256 is 32 bits, but I pass a string that is something longer to mcrypt, will only the first 32 bits be used? If this is the case, should I break up the string along the n bit boundaries into blocks and loop through each block of the key encrypting the previous output? I want to generate a random n bit key. Currently I'm looping through successive calls to mt_rand(0,1) and building the key (i.e. $binarykey .= mt_rand(1,0), then returning it w/ unpack("A$bits",$binarystring). There must be a better way, and is this even remotely a good way to do this, i.e. is it flawed? TIA! Please CC me since I'm not subscribed. Bill