mysqli prepared statements, insert NULL using bind params

Question

Does anyone know if it is possible to insert NULL into a column with MYSQLI bind_param.

I have a situation where sometimes I want to set a column to null in bind_param. Like so...

$column2 = "NULL";
$insert_data->bind_param('ss', $column1,$column2);

Obviously, this just writes NULL to the column as a string. I've tried null and \0 but they don't work.

I would like to know if there is some value I can make $column2 equal that will cause it to submit and actual null value instead.

Otherwise, I'll have to dynamically build the type and parameter lists and use call_user_func_array to create my binding such that columns aren't written if they are null. This I can do but it leads to a lot of fiddly extra code so I was just wondering if this can be avoided. Thanks