The following is defined in the code (PHP wrapper):

#if HAVE_LIBMCRYPT24
    MCRYPT_ENTRY2_2_4(ARCFOUR_IV, "arcfour-iv");
    MCRYPT_ENTRY2_2_4(ARCFOUR, "arcfour");
    MCRYPT_ENTRY2_2_4(BLOWFISH, "blowfish");
...
    MCRYPT_ENTRY2_2_4(MODE_CBC, "cbc");
    MCRYPT_ENTRY2_2_4(MODE_CFB, "cfb");
    MCRYPT_ENTRY2_2_4(MODE_ECB, "ecb");
    MCRYPT_ENTRY2_2_4(MODE_NOFB, "nofb");
    MCRYPT_ENTRY2_2_4(MODE_OFB, "ofb");
    MCRYPT_ENTRY2_2_4(MODE_STREAM, "stream");
    REGISTER_INI_ENTRIES();
#endif

So if MCRYPT_BLOWFISH works, then MCRYPT_MODE_CBC should work too.
I'm going to check this out later.

The following code snippet:

<?php
    echo MCRYPT_TWOFISH."<BR>\n";
    echo MCRYPT_MODE_CBC."<BR>\n";
?>

<?php
    define ("MODE1", MCRYPT_MODE_CBC);
    echo MODE1."<BR>\n";
?>

<?php
    define ("CIPHER", MCRYPT_TWOFISH);
    define ("MODE1", MCRYPT_MODE_CBC);
    define ("MODE2", MCRYPT_CBC);

    printf ("cipher=".CIPHER. " mode1=".MODE1. " mode2=". MODE2."<BR>\n");
?>

procudes this:

   twofish
   cbc
   cbc
   cipher=twofish mode1=cbc mode2=MCRYPT_CBC

Which is expected. From source code analyses it is not even possible that MCRYPT_TWOFISH is defined, but MCRYPT_MODE_CBC is not.

So closing