Warning: Cannot modify header

I’m getting multiple errors on the top of each website page that start with:
Warning: Cannot modify header information – headers already sent by (output started at…
I phoned Siteground, my host company, and they had the PHP set to version 7.0 (which is optimal). The only way they were able to remove the errors was to change it to PHP 5.6. I’ve done this as a temporary fix, but they said Cerber 7.0 wasn’t compatible with the optimal PHP. I am running Cerber Version 7.0 and it’s up to date. The insight in a similar question from months ago did not help. Thanks.