How do I get the value from a selected check box when I display it using a for loop?

I want to get the value and do calculations based off what the user selects.

<?php
                    $err = "";
            // I used an associative array because the items have a price that goes with them.
            $food = array("Bananas"=>"3", "Grapes"=>"4", "Cucumbers"=>"2","Carrots"=>"2");
            foreach($food as $f => $f_value){
                echo '<input type ="checkbox">'.$f." - $".$f_value.'<br>';
            }
            if (isset($_POST["$f_value"])) {
                echo $_POST['$f_value'];
            } else {
                $err = "Food has been selected";
            }
            ?>

When I select and item with the code I have it prints my error statement.