Since plesk 9.5, you can choose in plesk how php will run (as apache module will be executed as apache user, as cgi/fastcgi will be executed as domain user/system user)
wget -q -O - http://www.atomicorp.com/installers/atomic.sh | sh wget http://www.atomicrocketturtle.com/RPM-GPG-KEY.art.txt rpm --import RPM-GPG-KEY.art.txt
yum install mod_suphp
edit the /etc/httpd/conf.d/mod_suphp.conf and leave only LoadModule suphp_module modules/mod_suphp.so line
modify /etc/suphp.conf:
[global] logfile=/var/log/suphp.log loglevel=info webserver_user=apache docroot=/var/www/vhosts allow_file_group_writeable=false allow_file_others_writeable=false allow_directory_group_writeable=false allow_directory_others_writeable=false check_vhost_docroot=false errors_to_browser=false env_path=/bin:/usr/bin umask=0022 min_uid=500 min_gid=500 [handlers] x-httpd-php="php:/usr/bin/php-cgi" x-suphp-cgi="execute:!self"
create file /etc/httpd/conf.d/zzz_suphp.conf with content:
<Directory /var/www/vhosts>
<IfModule mod_suphp.c>
AddHandler x-httpd-php .php .php3 .php4 .php5 .phtml
suPHP_AddHandler x-httpd-php
suPHP_Engine on
</IfModule>
</Directory>
With this config you will enable the suPHP only for vhost folder and not for webmail and other folders.
You have also to check the file/folders permissions. If you have files/folders with 777 permission you will get a 500 error. You have to change it to 755 or 644.