Apache cannot be started on a Plesk server: Invalid command 'php_admin_value'

Symptoms

• The following error message is shown on the Home page in Plesk:

  PLESK_ERROR: New configuration files for the Apache web server were not created due to the errors in configuration templates: AH00526: Syntax error on line 2 of /var/www/vhosts/system/example.com/conf/vhost_ssl.conf: Invalid command 'php_admin_value', perhaps misspelled or defined by a module not included in the server configuration.

  The error can occur for different modules like php_flag, php_admin_flag.

• Apache cannot be started with the following error:

  CONFIG_TEXT: server.example.com httpd[13145]: AH00526: Syntax error on line 2 of /var/www/vhosts/system/example.com/conf/vhost.conf:
  server.example.com httpd[13145]: Invalid command 'php_admin_value', perhaps misspelled or defined by a module not included in the server configuration

Cause

The Apache module mod_php5 is not enabled.

Resolution

Enable the Apache module php5 by applying the steps 1 and 2 from this KB article.