Returns an array with the key sizes supported by the algorithm specified by the encryption descriptor. If it returns an empty array then all key sizes between 1 and mcrypt_enc_get_key_size() are supported by the algorithm.
<?php $td = mcrypt_module_open('rijndael-256', '', 'ecb', ''); var_dump(mcrypt_enc_get_supported_key_sizes($td));?>
This will print:
array(3) { [0]=> int(16) [1]=> int(24) [2]=> int(32) }