/usr/bin/php /var/www/forum/bin/phpbbcli.php user:add --username=test-user --password=test-password --email=mail@example.com

echo shell_exec('/usr/bin/php /var/www/forum/bin/phpbbcli.php user:add --username=test-user --password=test-password --email=mail@example.com');

[ERROR] The username you entered is too short.
     The password you entered is too short.
     The email address you entered is too short.