Warning: Undefined array key "series" in /home/httpd/vhosts/walking-tree.org/httpdocs/samples.php on line 12

Fatal error: Uncaught TypeError: Unsupported operand types: string * int in /home/httpd/vhosts/walking-tree.org/httpdocs/samples.php:155
Stack trace:
#0 /home/httpd/vhosts/walking-tree.org/httpdocs/pick_page.php(11): include()
#1 /home/httpd/vhosts/walking-tree.org/httpdocs/index.php(3): include('/home/httpd/vho...')
#2 {main}
  thrown in /home/httpd/vhosts/walking-tree.org/httpdocs/samples.php on line 155