Could not determine database connection parameters. Pass --db-url option when site-install
It appears, i passed one too few parameters to 'drush_invoke_process'. The documentation shows:

function drush_invoke_process($site_alias_record, $command_name, $commandline_args = array(), $commandline_options = array(),....)


In my case, I was not passing in the 'commandline_args' making the 'commandline_options' become 'commandline_args' instead of options including 'db-url'

Here, is the correct one:

$status = drush_invoke_process('@self', 'si', array('standard'), array(
                                                        'db-url' =&gt; 'mysql://'.$dbUser.':'.$dbPassword.'@'.$dbHost.':' . $dbPort . '/'.$dbName,
                                                        'account-pass' =&gt; $AdminPassword,
                                                   )
                              );


I hope this helps anyone