PHP 4.0.3pl1 worked fine, but I wanted to upgrade to 4.0.6 for the bug fixes. After upgrading, php pages will give a "Document contains no data" in Netscape and the logfile of Apache says : 
[Wed Jul 11 00:05:55 2001] [notice] child pid 14370 exit signal Segmentation fault (11), possible coredump in /usr/local/apache

I followed the instructions for gdb backtrace and this is what I get :

gdb /usr/local/apache/bin/httpd /tmp/core 
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This GDB was configured as "i386-redhat-linux"...
Core was generated by `/usr/local/apache/bin/httpd -DSSL'.
Program terminated with signal 11, Segmentation fault.
Reading symbols from /lib/libm.so.6...done.
Reading symbols from /lib/libcrypt.so.1...done.
Reading symbols from /lib/libdb.so.3...done.
Reading symbols from /lib/libdl.so.2...done.
Reading symbols from /lib/libc.so.6...done.
Reading symbols from /lib/ld-linux.so.2...done.
Reading symbols from /lib/libnss_files.so.2...done.
Reading symbols from /usr/lib/gconv/ISO8859-1.so...done.
Reading symbols from /usr/local/apache/libexec/libphp3.so...done.
Reading symbols from /usr/local/lib/libgd.so.3...done.
Reading symbols from /usr/lib/libpng.so.2...done.
Reading symbols from /usr/lib/libz.so.1...done.
Reading symbols from /usr/local/mysql/lib/mysql/libmysqlclient.so.6...done.
Reading symbols from /usr/lib/libgdbm.so.2...done.
Reading symbols from /lib/libpam.so.0...done.
Reading symbols from /lib/libresolv.so.2...done.
Reading symbols from /lib/libnsl.so.1...done.
Reading symbols from /usr/local/apache/libexec/libphp4.so...done.
Reading symbols from /usr/local/mysql/lib/mysql/libmysqlclient.so.10...done.
Reading symbols from /usr/lib/libttf.so.2...done.
Reading symbols from /usr/local/apache/libexec/mod_throttle.so...done.
#0  _dl_lookup_versioned_symbol (undef_name=0x8058a78 "setgid", ref=0xbffff904, symbol_scope=0x0, reference_name=0x40010c67 "", 
    version=0x400a32e0, reloc_type=7) at do-lookup.h:82
82      do-lookup.h: No such file or directory.
(gdb) bt
#0  _dl_lookup_versioned_symbol (undef_name=0x8058a78 "setgid", ref=0xbffff904, symbol_scope=0x0, reference_name=0x40010c67 "", 
    version=0x400a32e0, reloc_type=7) at do-lookup.h:82
#1  0x4000a83d in fixup (l=0x40013f30, reloc_offset=3367) at dl-runtime.c:81
#2  0x4000a9b0 in _dl_runtime_resolve () at dl-runtime.c:182
#3  0x80ae679 in child_main () at lstat64.c:44
#4  0x80aedcc in make_child () at lstat64.c:44
#5  0x80af146 in perform_idle_server_maintenance () at lstat64.c:44
#6  0x80af675 in standalone_main () at lstat64.c:44
#7  0x80afc43 in main () at lstat64.c:44
#8  0x400bc9cb in __libc_start_main (main=0x80af8ec <main>, argc=2, argv=0xbffffafc, init=0x8065e70 <_init>, 
    fini=0x8155c7c <_fini>, rtld_fini=0x4000aea0 <_dl_fini>, stack_end=0xbffffaf4) at ../sysdeps/generic/libc-start.c:92
(gdb)