MYSQL Updating multiple columns using variables

UPDATE syntax is different than INSERT syntax. An example of UPDATE would be:

"UPDATE products SET field1 = 'value1', field2 = '$val2', field3 = 5 WHERE sku = '$checksku'"  

Though this may be insecure. You should look into parameterized queries.

INSERT INTO products ($fields) VALUES ($values) ON DUPLICATE KEY UPDATE field = VALUES(field), ...

Don't forgot about unique or primary key