I have to agree that one should be using PHP 7.2. It also gives a nice performance boost to Matomo.
The required PHP version for Matomo is shown in [1] (5.5.9 or greater)
Can you please send me a link to the FAQ page mentioning 5.3 (e.g. to lukas@matomo.org) so they can be updated?
My mistake, I was skimming the docs only but overlooked the specially linked requirements. I would go further though and remove outdated PHP versions from that page and only recommend maintained versions.
Additionally, the error described by OP looks like autoloading is broken.
I have to agree that one should be using PHP 7.2. It also gives a nice performance boost to Matomo. The required PHP version for Matomo is shown in [1] (5.5.9 or greater) Can you please send me a link to the FAQ page mentioning 5.3 (e.g. to lukas@matomo.org) so they can be updated?
[1] https://matomo.org/docs/requirements/