Error displaying the error page: Application Instantiation Error:

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.7.0
-- PHP version -- : 5.6.3
-- Browser(s) name and version -- : Firefox
-- Error-message(debug-mod must be tuned on) -- : Error displaying the error page: Application Instantiation Error: Could not connect to MySQL.

Hi, After updating from easyapache3 to 4 I no longer cannot connect to the database from the front end, backend is ok and can connect to everything?

I get this error message 'Error displaying the error page: Application Instantiation Error: Could not connect to MySQL'

I have 4 sites on the server and 2 work ok and 2 don't with the same error message? I have done the obvious with the config file and mySQL everything appears to be ok in there so wondering if there is something in Apache that is not activated that should be?

I also get this message after pressing refresh 'Fatal error: Call to undefined function simplexml_load_file() in /home/xxxxx/public_html/libraries/joomla/language/helper.php on line 576'

**cannot find a solution so have reverted back to easyapache3 and everything works again** any clues as to what could be causing this behaviour?

Hoping you can help
Thanks