Here's the configure line I used:

./configure --with-apxs=/usr/local/apache/bin/apxs --with-ldap=/usr/local --with-gdbm=/usr/local --with-mysql=/usr/local/mysql  --enable-track-vars --enable-sysvshm  --enable-force-cgi-redirect --enable-ftp --with-gd  --with-png-dir=/usr/local --with-xpm-dir=/usr/local  --with-gettext --with-jpeg-dir=/usr/local --with-zlib --with-tiff-dir=/usr/local --with-snmp=/usr/local --enable-ucd-snmp-hack --with-freetype-dir=/usr/local

Okay..try this:

# gdb sapi/cli/php
(gdb) run
(gdb) bt

(dunno if you have gdb installed..but use the debugger you have there)

Output from gdb:

porn$ gdb sapi/cli/php
GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "sparc-sun-solaris2.8"...(no debugging symbols found)...
(gdb) run
Starting program: /usr/home/mike/www/php-4.3.4/sapi/cli/php
(no debugging symbols found)...(no debugging symbols found)...warning: Lowest section in /usr/lib/libintl.so.1 is .hash at 00000074
(no debugging symbols found)...Assertion failed: BER_VALID( ber ), file encode.c, line 471
                                                                                                                                           
Program received signal SIGABRT, Aborted.
0xfed9f8d8 in _libc_kill () from /usr/lib/libc.so.1
(gdb) bt
#0  0xfed9f8d8 in _libc_kill () from /usr/lib/libc.so.1
#1  0xfed35944 in abort () from /usr/lib/libc.so.1
#2  0xfed35be8 in _assert () from /usr/lib/libc.so.1
#3  0xff323c88 in ber_start_seq () from /usr/local/lib/liblber.so.2
#4  0xfeb2b52c in __BP_ber_printf () from /usr/lib/sldaputil.so.5
#5  0xfeb431b4 in simple_bind_nolock () from /usr/lib/sldaputil.so.5
#6  0xfec424f0 in openConnection () from /usr/lib/libsldap.so.1
#7  0xfec41998 in makeConnection () from /usr/lib/libsldap.so.1
#8  0xfec434cc in __s_api_getConnection () from /usr/lib/libsldap.so.1
#9  0xfec39b30 in get_current_session () from /usr/lib/libsldap.so.1
#10 0xfec3a8e0 in search_state_machine () from /usr/lib/libsldap.so.1
#11 0xfec3b3d8 in __ns_ldap_list () from /usr/lib/libsldap.so.1
#12 0xfec6a444 in _nss_ldap_lookup () from /usr/lib/nss_ldap.so.1
#13 0xfec69a6c in getbyname () from /usr/lib/nss_ldap.so.1
#14 0xfed49554 in nss_search () from /usr/lib/libc.so.1
#15 0xfefb9c08 in _switch_getservbyname_r () from /usr/lib/libnsl.so.1
#16 0xfef9b63c in _get_hostserv_inetnetdir_byname () from /usr/lib/libnsl.so.1
#17 0xff045c34 in getservbyname_r () from /usr/lib/libsocket.so.1
#18 0xfeec24c8 in _init_snmp () from /usr/local/lib/libnetsnmp.so.5
#19 0xfeeb5298 in init_snmp () from /usr/local/lib/libnetsnmp.so.5
#20 0xd9064 in zm_startup_snmp ()
#21 0x19c388 in zend_startup_module ()
#22 0x167f48 in php_startup_extensions ()
#23 0x1684dc in php_module_startup ()
#24 0x1ae1b4 in main ()
(gdb)

Does it work if you compile php without --with-ldap ?
Does it work if you compile php without --with-snmp ?

And what net-snmp version do you have there? 
This actually is caused by snmp, not ldap..but I'd like
to know whether that linkage with openldap causes this.

btw. This block in the backtrace seems a bit weird:

#3  0xff323c88 in ber_start_seq () from /usr/local/lib/liblber.so.2
#4  0xfeb2b52c in __BP_ber_printf () from /usr/lib/sldaputil.so.5

Notice how that 'sldaputil' calls function in openldap?
Solaris has it's own ldap libraries, you actually should use those instead, so just remove the openldap installation,
and configure PHP from fresh sources with plain --with-ldap

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