Update PHP to version PHP 5.2.5 while running Plesk? Horde WebMail broken? Log in and become the root system user and run this one-liner, then restart Apache:

# cat > /etc/httpd/conf.d/zz050a_horde_php_patch.conf <<EOF 
<DirectoryMatch /usr/share/psa-horde>
    php_admin_value include_path "/usr/share/psa-horde/lib:/usr/share/psa-horde:/usr/share/psa-horde/pear:."
</DirectoryMatch>
EOF
# service httpd configtest && service httpd graceful

Some discussion:

PHP 5.2.5 introduced a security fix to not allow scripts to override