it looks like you will have to recompile php otherwise you could upgrade to 2.2.3 and while you are compiling you will come to the configure part use something like this:
download the lastest version of php into your /usr/local/src directory.
in shell type:
tar -xzf php-4.x.x.tar.gz
change into the php directory
cd php-4.x.x
now for the configuring
in shell type this:
./configure \
--with-mcrypt
you may need to do some more configure options i dont know
you must compile mcrypt with the --disable-posix-threads configure option
only prob is i dont know what you compile first _