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

Symptoms

1. Error on Plesk home page:

   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.

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

2. Apache cannot start:

   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

3. The domain example.com was migrated from a server with mod_php5 enabled.

Cause

Apache module mod_php5 is disabled for security reasons.

Resolution

1. Log in to Plesk via SSH
2. Open vhost.conf mentioned in the error message and comment out the string with  'php_admin_value' directive.