The exact value of path variables can be known from /etc/psa/psa.conf file on Parallels Plesk Panel server. Below is example of /etc/psa/psa.conf file:
Plesk tree
PRODUCT_ROOT_D /usr/local/psa
Directory of SysV-like Plesk initscripts
PRODUCT_RC_D /etc/init.d
Directory for config files
PRODUCT_ETC_D /usr/local/psa/etc
Virtual hosts directory
HTTPD_VHOSTS_D /var/www/vhosts
Apache configuration files directory
HTTPD_CONF_D /etc/httpd/conf
Apache include files directory
HTTPD_INCLUDE_D /etc/httpd/conf.d
Apache binary files directory
HTTPD_BIN_D /usr/bin
Apache log files directory
HTTPD_LOG_D /var/log/httpd
Apache startup script
HTTPD_SERVICE httpd
Qmail directory
QMAIL_ROOT_D /var/qmail
Location of qmail maildirs
QMAIL_MAILNAMES_D /var/qmail/mailnames
Path to rblsmtpd
RBLSMTPD /usr/sbin/rblsmtpd
Courier-IMAP
COURIER_IMAP_ROOT_D /
Proftpd
FTPD_CONF /etc/proftpd.conf
FTPD_CONF_INC /etc/proftpd.include
FTPD_BIN_D /usr/bin
FTPD_VAR_D /var/run/proftpd
FTPD_SCOREBOARD /var/run/proftpd/scoreboard
Bind
NAMED_RUN_ROOT_D /var/named/run-root
Webalizer
WEB_STAT /usr/bin/webalizer
Logrotate
LOGROTATE /usr/local/psa/logrotate/sbin/logrotate
MySQL
MYSQL_VAR_D /var/lib/mysql
MYSQL_BIN_D /usr/bin
PostgreSQL
PGSQL_DATA_D /var/lib/pgsql/data
PGSQL_BIN_D /usr/bin
Backups directory
DUMP_D /var/lib/psa/dumps
Mailman directories
MAILMAN_ROOT_D /usr/lib/mailman
MAILMAN_VAR_D /var/lib/mailman
Python binary
PYTHON_BIN /usr/bin/python2.4
Tomcat root directory
CATALINA_HOME /usr/share/tomcat5
DrWeb
DRWEB_ROOT_D /opt/drweb
DRWEB_ETC_D /etc/drweb
GnuPG binary
GPG_BIN /usr/bin/gpg
Tar binary
TAR_BIN /bin/tar
Curl certificates
CURL_CA_BUNDLE_FILE /usr/share/curl/curl-ca-bundle.crt
AWStats
AWSTATS_ETC_D /etc/awstats
AWSTATS_BIN_D /var/www/cgi-bin/awstats
AWSTATS_TOOLS_D /usr/share/awstats
AWSTATS_DOC_D /var/www/html/awstats
openssl binary
OPENSSL_BIN /usr/bin/openssl
LIB_SSL_PATH /lib/libssl.so.5
LIB_CRYPTO_PATH /lib/libcrypto.so.5
CLIENT_PHP_BIN /usr/local/psa/bin/php-cli
