How to insert an actual NULL value into a nullable column?

Question

function save($gmt, $name, $address, $phone, $remark)
{
    $query= "INSERT INTO `user` (`gmt`, `name`, `address`, `phone`, `remark`) VALUES ('$gmt', '$name', '$address', '$phone', '$remark')";
    mysql_query($query);
}

Here, address, phone, and remark can be NULL. I need it to save NULL whenever the variable is set to NULL and the column is nullable, instead of inserting an empty string.

How can I insert NULL value into the database using PHP?