mcrypt_enc_get_modes_name

<?php
    $td = mcrypt_module_open (MCRYPT_CAST_256, '', MCRYPT_MODE_CFB, '');
    echo mcrypt_enc_get_modes_name($td). "\n";
  
    $td = mcrypt_module_open ('cast-256', '', 'ecb', '');
    echo mcrypt_enc_get_modes_name($td). "\n";
?>

Печатает:
CFB
ECB