php syntax for variable column name in query

php syntax for variable column name in query

I am trying to pass the name of a column as a variable to a PHP database
query.
the hard coded syntax works perfectly and is: select max([257612cr]) as
price from Price_TekwaniPrice where customeraccount='DAY001'
When I pass the variable I get error trying to get property ofnon object.
my syntax is:
$query = $this->db->query("
select max(['$product']) as price from Price where
customeraccount='$customer'
");
I also tried:
$query = $this->db->query("
select max(".$product.") as price from Price where
customeraccount='$customer'
");
I have confirmed that the variables are being passed correctly. the syntax
for '$customer' works perfectly so just passing the $product variable as a
column name is proving cumbersome.
I am using php with codeigniter. any advice welcome.
Thanks as always,