Are you able to reproduce it under a different OS?
Or at least with different glibc?
Is it reproducible only with Apache2 or with CLI too?
As far as I can see, mysql is built as shared module or am I wrong?

I did't try in other OS. Later, I'll see in Windows - but 
I have to set up the environment first. 
Yes. I used glibc 2.3.4 before, and switched to 2.3.5 to 
see if it helps. 
It also happened earlier, when I had some older glibc, but 
I ignored it. 
The code also fails in CLI. Actually, we test it in CLI 
because Apache doesn't get any output from PHP module 
since it dies - proxy says that zero-sized reply comes. 
And finally: Yes, we build as much we can as modules to 
package it into separate packages.

Works fine on SuSE 9.2/glibc 2.3.3. 
I'm 100% sure it also works on other systems I have around here, but I'll check it tomorrow.

Using this code, from the url you posted:

<?php
  $link = mysql_pconnect('localhost', 'aaa', 'bbb');
  mysql_select_db('ccc', $link);
  if($link) mysql_close($link);
  echo 'I am still alive';
?>

And I can not reproduce this. Not with PHP 4.3.12-dev, PHP 5.1-dev..and I have glibc 2.3.4 (FC2)

What is the configure line you are using with PHP?

Our Configure Command: 
'./configure' 'LDFLAGS=' 'CFLAGS=-O2 -march=i686 -DEAPI=1 
-I/usr/X11R6/include -I/usr/include/apr 
-I/usr/include/apr-util -I/usr/include' 'CXXFLAGS=-O2 
-march=i686' 'FFLAGS=-O2 -march=i686' 'CPPFLAGS=' 
'CC=i686-pld-linux-gcc' 'CXX=i686-pld-linux-g++' 
'--build=i686-pld-linux' '--prefix=/usr' 
'--exec-prefix=/usr' '--bindir=/usr/bin' 
'--sbindir=/usr/sbin' '--sysconfdir=/etc/php' 
'--datadir=/usr/share' '--includedir=/usr/include' 
'--libdir=/usr/lib' '--libexecdir=/usr/lib' 
'--localstatedir=/var' '--sharedstatedir=/var/lib' 
'--mandir=/usr/share/man' '--infodir=/usr/share/info' 
'--x-libraries=/usr/X11R6/lib' 
'--with-apxs2=/usr/sbin/apxs' '--enable-maintainer-zts' 
'--with-config-file-path=/etc/php' 
'--with-exec-dir=/usr/bin' '--disable-debug' 
'--enable-memory-limit' '--enable-bcmath=shared' 
'--enable-calendar=shared' '--enable-ctype=shared' 
'--enable-dba=shared' '--enable-dbx=shared' 
'--enable-dio=shared' '--enable-dom=shared' 
'--enable-exif=shared' '--enable-filepro=shared' 
'--enable-ftp=shared' '--enable-gd-native-ttf' 
'--enable-gd-jus-conf' '--enable-magic-quotes' 
'--enable-mbstring=shared,all' '--enable-mbregex' 
'--enable-pcntl=shared' '--enable-posix=shared' 
'--enable-session' '--enable-shared' 
'--enable-shmop=shared' '--enable-sysvmsg=shared' 
'--enable-sysvsem=shared' '--enable-sysvshm=shared' 
'--enable-track-vars' '--enable-trans-sid' 
'--enable-safe-mode' '--enable-sockets=shared' 
'--enable-ucd-snmp-hack' '--enable-wddx=shared' 
'--enable-xml=shared' '--enable-yp=shared' 
'--enable-soap=shared' '--with-bz2=shared' 
'--with-cpdflib=shared' '--with-curl=shared' '--with-db4' 
'--with-dbase=shared' '--with-expat-dir=shared,/usr' 
'--with-iconv=shared' '--with-fam=shared' 
'--with-filepro=shared' '--with-freetype-dir=shared' 
'--with-gettext=shared' '--with-gd=shared,/usr' 
'--with-gdbm' '--with-gmp=shared' '--with-imap=shared' 
'--with-imap-ssl' '--with-interbase=shared,/usr' 
'--with-jpeg-dir=/usr' '--with-ldap=shared' 
'--with-mcrypt=shared' '--with-mhash=shared' 
'--with-mime-magic=shared,/usr/share/file/magic.mime' 
'--with-ming=shared' '--with-mnogosearch=shared,/usr' 
'--with-msession=shared' '--with-mssql=shared' 
'--with-mysql=shared,/usr' 
'--with-mysql-sock=/var/lib/mysql/mysql.sock' 
'--with-mysqli=shared' '--with-ncurses=shared' 
'--with-openssl=shared' '--with-pcre-regex=shared' 
'--with-pear=/usr/share/pear' '--with-pgsql=shared,/usr' 
'--with-png-dir=/usr' '--with-pspell=shared' 
'--with-readline=shared' '--with-recode=shared' 
'--with-regex=php' '--without-sablot-js' 
'--with-snmp=shared' '--with-sybase=shared,/usr' 
'--with-sybase-ct=shared,/usr' '--with-sqlite=shared,/usr' 
'--with-t1lib=shared' '--with-tidy=shared' 
'--with-tiff-dir=/usr' '--with-unixODBC=shared' 
'--with-xmlrpc=shared,/usr' '--with-xsl=shared' 
'--with-zlib=shared' '--with-zlib-dir=shared,/usr'

If you load only and ONLY the mysql extension in your php.ini, can you reproduce this?

I can't really test it at the moment. My colleague also 
encountered the same problem after upgrade to glibc 2.3.5 
and PHP 5.0.4 - when he downgraded to PHP 5.0.3 everything 
was working fine. 
I searched the php bugs database for 
"plist_entry_destructor" and I found that one user had 
similar problem in PHP 4.3.x some time ago, and it makes 
me think that this is not only mysql-module related. 
I suggest you to try the new glibc and see if PHP works 
without any problems. If there is anything else I can do, 
just ask. Tomorrow, we will be trying to find the bug in 
the PHP code, so I might have some more information in a 
day or two.

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

Successful reproduction.

Running through the CLI,
the following works fine:
<?php
    $conn = pg_connect('dbname=lod user=lod');
?>

the following will segfault on termination:
<?php
    $conn = pg_pconnect('dbname=lod user=lod');
?>

Running Debian Sarge.
libc-2.3.2
Fresh cvs version of PHP
php -v:
PHP 5.1.0-dev (cli) (built: Jul  6 2005 10:55:39)
Copyright (c) 1997-2005 The PHP Group
Zend Engine v2.1.0-dev, Copyright (c) 1998-2004 Zend Technologies

stack trace:
#0  0xb7c4d900 in ?? ()
#1  0x0816af31 in plist_entry_destructor (ptr=0x8337578)
    at /home/lod/Downloads/php-cvs/Zend/zend_list.c:210
#2  0x081699d8 in zend_hash_apply_deleter (ht=0x826cd18, p=0x833d260)
    at /home/lod/Downloads/php-cvs/Zend/zend_hash.c:574
#3  0x08169a7c in zend_hash_graceful_reverse_destroy (ht=0x826cd18)
    at /home/lod/Downloads/php-cvs/Zend/zend_hash.c:640
#4  0x08161bef in zend_shutdown (tsrm_ls=0x0) at /home/lod/Downloads/php-cvs/Zend/zend.c:713
#5  0x081229fe in php_module_shutdown (tsrm_ls=0x8268018)
    at /home/lod/Downloads/php-cvs/main/main.c:1558
#6  0x081dc959 in main (argc=1, argv=0xbffffba4) at /home/lod/Downloads/php-cvs/sapi/cli/php_cli.c:1148