Drop down ranges to text box ranges how to keep secure

I am changing a form that uses values in two drop down form fields to enter a numeric range into a database

I want to change it so that the user cna enter two values into text fields instead

what do I need to do to these vaules before i put them in my sql query

 

Sanitise the variable for MySQL:

$var = mysql_real_escape_string($_POST["textbox"]);

PHP:

Or MySQLi:

$var = mysqli_real_escape_string($_POST["textbox"]);

PHP:

Then just use that variable in the query.