Installer.php error on line 108

Hi,
I have uploaded the installer.php to the web server along with the database archive.

When I navigate to the file, it appears as a 404. When I tail the nginx logs, it says:

2018/07/02 21:05:15 [error] 10043#0: *1 FastCGI sent in stderr: “PHP message: PHP Fatal error: Uncaught Error: Call to undefined function json_decode() in /var/www/html/installer.php:108
Stack trace:
#0 {main}
thrown in /var/www/html/installer.php on line 108” while reading response header from upstream, client: xxxxxxx, server: _, request: “GET /installer.php HTTP/1.1”, upstream: “fastcgi://unix:/run/php-fpm/www.sock:”, host: “domain”
2018/07/02 21:05:15 [error] 10043#0: *1 open() “/var/www/html/50x.html” failed (2: No such file or directory), client: xxxxxxx, server: _, request: “GET /installer.php HTTP/1.1”, upstream: “fastcgi://unix:/run/php-fpm/www.sock”, host: “domain”

All other PHP works fine on this server.

Any help?
Thanks