The mcrypt support in PHP 3.0 is difficult to get working, because it depends on a specific version of libmcrypt but there's no indication in the documentation which version this is.  Using libmcrypt 2.3.4, the build errors out with:

gcc -fPIC -O2 -march=pentiumpro -fno-strength-reduce -DLINUX=2 -DNO_DBM_REWRITEMAP -DMOD_SSL=205100 -DEAPI -DUSE_EXPAT -I../lib/expat-lite -fpic -DSHARED_MODULE  -I. -I.   -I/usr/include/apache                  -I/usr/include/freetype -I/usr/include/mysql -I/usr/include/ucd-snmp    -c functions/mcrypt.c -o functions/mcrypt.o
functions/mcrypt.c: In function `php_minit_mcrypt':
functions/mcrypt.c:166: `MCRYPT_RC6_256' undeclared (first use in this function)
functions/mcrypt.c:166: (Each undeclared identifier is reported only once
functions/mcrypt.c:166: for each function it appears in.)
functions/mcrypt.c:168: `MCRYPT_RC6_128' undeclared (first use in this function)
functions/mcrypt.c:169: `MCRYPT_RC6_192' undeclared (first use in this function)
make: *** [functions/mcrypt.o] Error 1

Trying to build with libmcrypt 2.4.0 is even less successful, with very few of the ciphers in functions/mcrypt.c found in the mcrypt header files.

It would be useful if php would check for a given cipher in mcrypt.h *before* attempting to declare it as a PHP constant.  Here is a short patch which works tfor PHP 3.0.15 with libmcrypt 2.3.4.  This patch should probably be generalized to accomodate other additions or subtractions from the libmcrypt API.

diff -uNr php-3.0.15-orig/functions/mcrypt.c php-3.0.15/functions/mcrypt.c
--- php-3.0.15-orig/functions/mcrypt.c  Fri Dec 31 22:31:16 1999
+++ php-3.0.15/functions/mcrypt.c       Thu Mar 30 09:16:09 2000
@@ -163,10 +163,16 @@
        MCRYPT_ENTRY2(RIJNDAEL_256);
        MCRYPT_ENTRY2(RC2_256);
        MCRYPT_ENTRY2(RC2_128);
+#ifdef MCRYPT_RC6_256
        MCRYPT_ENTRY2(RC6_256);
+#endif
        MCRYPT_ENTRY2(IDEA);
+#ifdef MCRYPT_RC6_128
        MCRYPT_ENTRY2(RC6_128);
+#endif
+#ifdef MCRYPT_RC6_192
        MCRYPT_ENTRY2(RC6_192);
+#endif
        MCRYPT_ENTRY2(RC4);
 #else
 #error Please update your mcrypt library


Barring this, it would be helpful if the PHP documentation stated clearly *which* version of libmcrypt is supposed to be used.