$config = '
#Start User username1
<VirtualHost IP:80>
  ServerName username1.someserver
  ServerAlias www.username1.someserver
  DocumentRoot /var/www/username1/
   <Directory /var/www/username1>
     AllowOverride AuthConfig FileInfo Indexes Limit Options
   </Directory>
</VirtualHost>
#End User username1';

echo preg_replace('/#Start User username1.*#End User username1/s', '', $config);