chasiv does not know the mysqli interface. I don't know it either, but I checked the PHP manual and:
$rownews = mysqli_query($cxn, $execute);
is correct if $cxn is your connection link resource.
The problem is that mysqli_query() returns a result resource if the query succeeded. It returns false if it failed, e.g., because your SQL statement is syntactically incorrect, refers to nonexistent table or column names, etc.
Consequently, $rownews could contain a false value, so you have to check that it does not contain false before using it with mysqli_fetch_assoc().