How can show SQL query in php [duplicate]

I have this query with this php code and can't show the result in echo, I can convert to string?

//conexion is my function to conect to DB and is done


$costTotal=conexion();
$costTotal=$costTotal->query("SELECT SUM(product_cost) FROM product");

echo $costTotal;

For example expected string(5)"18000"

source https://stackoverflow.com/questions/76916208/how-can-show-sql-query-in-php