Cannot connect PHP to SQL Server in no way
This is NOT 18456 (login failed). The error says


  Server is not found or not accessible


And it means that you should control that:


SQL Server is started and running
you use the correct instance name (so show us your connection
string), and if the instance is named, SQL Server Browser also should
be running
TCP\IP is enabled


You cannot find anything in error log because the server was not reached at all.