Configure line:

./configure --with-mysql --with-apache=../apache_1.3.1 --enable-track-vars

Trying to compile php3 as a module for Apache 1.3.1, get the following link error:

        cc  -DUW=700 -DUSE_HSREGEX -DSERVER_SUBVERSION=\"PHP/3.0.3\" `./apaci` -o httpd buildmark.o modules.o  modules/standard/libstandard.a  modules/php3/libphp3.a  main/libmain.a ./os/unix/libos.a ap/libap.a regex/libregex.a -Lmodules/php3 -L../modules/php3 -L../../modules/php3 -lmodphp3 -L/usr/local/lib/mysql -lmysqlclient -lm -lsocket -lresolv -lsocket -lnsl -lcrypt -lgen
Undefined                       first referenced
 symbol                             in file
alloca                              modules/php3/libphp3.a(configuration-parser.tab.o)
UX:ld: ERROR: httpd: fatal error: Symbol referencing errors. No output written to httpd

Unixware 7 only provides the alloca() function via the ucb compatibility package/compiler, but it is not recommended to mix the ucb libraries with the standard C libraries.  The config.h file hints at there being an alloca.c, but there is none in the package:

/* Define if using alloca.c.  */
#define C_ALLOCA 1