500 Error with no details on site migration I'm trying to migrate my site from a PHP5.6 host to a PHP 7 host in preparation for Craft 3.

I've copied everything across and am trying to load an exact copy of the production site on a different domain to see if it's all working as expected.

Unfortunately I don't make it far.
On hitting the site I get:

PHP Fatal error:  Uncaught Error: Call to a member function run() on integer in /blah.com.au/htdocs/craft/app/index.php:62

Stack trace:
#0 /blah.com.au/htdocs/public/index.php(19): require_once()
#1 {main}
thrown in /blah.com.au/htdocs/craft/app/index.php on line 62


There are no other details anywhere.
To avoid cache issues I've also done a fresh upload of Craft and then copied across my config, templates, plugins, public directories etc.
Exact same error.

I've updated the config files (db/general) to have the correct paths for this new domain. The database is the same as production.

my .htaccess shouldn't be causing issues.

Any ideas peoples?
It turns out my MySQL wasn't installed correctly.
I'm using RDS but my setup didn't meet the basic requirements to connect to it from PHP.