Once you get an array of values rather than just a single value you will need to make use of php array functions to manipulate the data. If you would like to just print out the values from the array in a comma separated list you could do so using the implode function like this:
implode(", ", $_POST["PropertyType"])
You can do many things with arrays in php, you should take some time to review the implode function and some others for manipulating the arrays to get an idea for the things you can do.
php.net/implode