Perl's DBI works fine... :)
php.ini was not changed from the distribution.

% tail /var/www/logs/error_log
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[Sun Sep 10 22:59:17 2000] [notice] Apache/1.3.12 (Unix) PHP/4.0.2 mod_perl/1.24 mod_ssl/2.6.2 OpenSSL/0.9.5a configured -- resuming normal operations
/usr/libexec/ld.so: Undefined symbol "_mysql_init" called from httpd:/usr/lib/apache/modules/libphp4.so at 0x404112b4

% ldconfig -r
/var/run/ld.so.hints:
        search directories: /usr/lib:/usr/local/lib/mysql:/usr/local/lib:/usr/X11R6/lib
        0:-lkrb.8.0 => /usr/lib/libkrb.so.8.0
        1:-lkvm.5.1 => /usr/lib/libkvm.so.5.1
        2:-lm.0.1 => /usr/lib/libm.so.0.1
        3:-lobjc.0.0 => /usr/lib/libobjc.so.0.0
        4:-lpcap.1.1 => /usr/lib/libpcap.so.1.1
        5:-lpanel.1.2 => /usr/lib/libpanel.so.1.2
        6:-lg2c.5.24 => /usr/lib/libg2c.so.5.24
        7:-lossaudio.0.0 => /usr/lib/libossaudio.so.0.0
        8:-lperl.6.0 => /usr/lib/libperl.so.6.0
        9:-lssl.2.3 => /usr/local/lib/libssl.so.2.3
        10:-lstdc++.30.0 => /usr/lib/libstdc++.so.30.0
        11:-ltermcap.7.1 => /usr/lib/libtermcap.so.7.1
        12:-ltermlib.7.1 => /usr/lib/libtermlib.so.7.1
        13:-ledit.0.0 => /usr/lib/libedit.so.0.0
        14:-lkdb.6.0 => /usr/lib/libkdb.so.6.0
        15:-lwrap.1.1 => /usr/lib/libwrap.so.1.1
        16:-lform.1.1 => /usr/lib/libform.so.1.1
        17:-lz.1.3 => /usr/lib/libz.so.1.3
        18:-ltk83.1.3 => /usr/local/lib/libtk83.so.1.3
        19:-lskey.1.0 => /usr/lib/libskey.so.1.0
        20:-lcrypto.2.3 => /usr/local/lib/libcrypto.so.2.3
        21:-lICE.6.3 => /usr/X11R6/lib/libICE.so.6.3
        22:-lSM.6.0 => /usr/X11R6/lib/libSM.so.6.0
        23:-lmenu.1.1 => /usr/lib/libmenu.so.1.1
        24:-lXIE.6.0 => /usr/X11R6/lib/libXIE.so.6.0
        25:-lXaw.6.1 => /usr/X11R6/lib/libXaw.so.6.1
        26:-lXext.6.3 => /usr/X11R6/lib/libXext.so.6.3
        27:-lotermcap.3.0 => /usr/lib/libotermcap.so.3.0
        28:-lmysqlclient.6.0 => /usr/local/lib/mysql/libmysqlclient.so.6.0
        29:-locurses.3.0 => /usr/lib/libocurses.so.3.0
        30:-lc_r.3.0 => /usr/lib/libc_r.so.3.0
        31:-lXi.6.0 => /usr/X11R6/lib/libXi.so.6.0
        32:-lXmu.6.0 => /usr/X11R6/lib/libXmu.so.6.0
        33:-lXpm.4.11 => /usr/X11R6/lib/libXpm.so.4.11
        34:-lintl.1.0 => /usr/local/lib/libintl.so.1.0
        35:-lutil.4.5 => /usr/lib/libutil.so.4.5
        36:-lPEX5.6.0 => /usr/X11R6/lib/libPEX5.so.6.0
        37:-ldes.6.0 => /usr/lib/libdes.so.6.0
        38:-liberty.3.0 => /usr/lib/libiberty.so.3.0
        39:-lXp.6.2 => /usr/X11R6/lib/libXp.so.6.2
        40:-lcurses.7.1 => /usr/lib/libcurses.so.7.1
        41:-lkafs.6.0 => /usr/lib/libkafs.so.6.0
        42:-lkadm.6.0 => /usr/lib/libkadm.so.6.0
        43:-lXt.6.0 => /usr/X11R6/lib/libXt.so.6.0
        44:-lc.25.0 => /usr/lib/libc.so.25.0
        45:-lacl.5.0 => /usr/lib/libacl.so.5.0
        46:-ltcl83.1.3 => /usr/local/lib/libtcl83.so.1.3
        47:-lcurses++.1.1 => /usr/lib/libcurses++.so.1.1
        48:-lXtst.6.1 => /usr/X11R6/lib/libXtst.so.6.1
        49:-lX11.6.1 => /usr/X11R6/lib/libX11.so.6.1
        50:-loldX.6.0 => /usr/X11R6/lib/liboldX.so.6.0

% nm /usr/local/lib/mysql/mysqlclient.so.6.0
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00002d28 T _mysql_get_client_info
00002d44 T _mysql_get_host_info
00002d54 T _mysql_get_proto_info
00002d18 T _mysql_get_server_info
00002980 T _mysql_init
00002c44 T _mysql_kill
000022b8 T _mysql_list_dbs
00002484 T _mysql_list_fields
00002740 T _mysql_list_processes
000023a4 T _mysql_list_tables
00000b28 t _mysql_once_init
00002d64 T _mysql_options
.
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.Compiled-in modules:
  http_core.c
  mod_env.c
  mod_log_config.c
  mod_mime.c
  mod_negotiation.c
  mod_status.c
  mod_include.c
  mod_autoindex.c
  mod_dir.c
  mod_cgi.c
  mod_asis.c
  mod_imap.c
  mod_actions.c
  mod_userdir.c
  mod_alias.c
  mod_access.c
  mod_auth.c
  mod_so.c
  mod_setenvif.c
  mod_ssl.c
suexec: disabled; invalid wrapper /usr/sbin/suexec

if /usr/local/lib/mysql is not part of ldconfig at boot-time even perl's DBI complains about libmysqlclient.so.6.0

cd /usr/local/lib; ln -s mysql/*
does not fix the problem.