PHP 4.0.6 doesn't recognize Caldera's OpenUNIX 8.  Replacing the 
supplied config.guess/config.sub with the ones from ftp://ftp.gnu.org/pub/gnu/config/ seems to fix it. 

The supplied one returns:
$ sh config.guess
config.guess: unable to guess system type

This script, last modified 2001-04-20, has failed to recognize
the operating system you are using. It is advised that you
download the most up to date version of the config scripts from

    ftp://ftp.gnu.org/pub/gnu/config/

If the version you run (config.guess) is already up to date, please
send the following data and any information you think might be
pertinent to <config-patches@gnu.org> in order to provide the needed
information to handle your system.

config.guess timestamp = 2001-04-20

uname -m = i386
uname -r = 5
uname -s = OpenUNIX
uname -v = 8.0.0

/usr/bin/uname -p = x86at
/bin/uname -X     = 
System = OpenUNIX
Node = lerami
Release = 5
KernelID = 01/06/09
Machine = Pentium III
BusType = ISA
Serial = 6ID015378
Users = 40
OEM# = 0
Origin# = 1
NumCPU = 1

hostinfo               = 
/bin/universe          = 
/usr/bin/arch -k       = 
/bin/arch              = 
/usr/bin/oslevel       = 
/usr/convex/getsysinfo = 

UNAME_MACHINE = i386
UNAME_RELEASE = 5
UNAME_SYSTEM  = OpenUNIX
UNAME_VERSION = 8.0.0
$ 
The one from the ftp site above returns:
$ sh config.guess
i686-unknown-sysv5OpenUNIX8.0.0
$ 

I did build PHP, and the only problem I had was libtool not handling dependencies.  I changed libtool to use pass_all for dependency type, and it worked, modulo the warning from cc about including -lc, but I couldn't find an easy way to deal with that.  the libphp4.so works however.