Update.php generates a syntax error

I try to run tt-rss on a Synology Diskstation, DSM 6.2.6. Newer versions of this might support Docker installations; mine doesn’t. (Or am I just too stupid to find it?)
The installed php version is 7.4. I run update.php on the CLI like this:

sudo su -s /bin/sh -c "/usr/local/bin/php74 -f /volume1/web/tt-rss/update.php -- --feeds --debug-force-refetch" http

I get an error for every feed it’s trying to update:

PHP Parse error:  syntax error, unexpected ':', expecting '{' in /volume1/web/tt-rss/update.php on line 23

For it’s own purposes Synology has a php 5.6 which is called “php”. Is it possible that somehow this old version is called although I specify php74 on the CLI?

https://dev.tt-rss.org/tt-rss/tt-rss/src/branch/master/update.php#L23
https://www.php.net/manual/en/language.types.declarations.php

It does seem likely you’re not actually using 7.4.

https://srv.tt-rss.org/ttrss-docs/classes/Config.html#constant_PHP_EXECUTABLE
Tiny Tiny RSS – Global configuration ctrl-f ‘minimal config’

Yes, that was it. Thank you!