$_GET value into if else statement

Question

I have a dropdown list in a form, which holds the values of the options. Once submitted I want to change the query based on which option I chose.

My form:

<form method="get" name="datum" id="datum-form" onchange="document.getElementById('datum-form').submit()">
    <select id="datum" class="form-control">
         <option value="">Select Filter</option>
         <option value="0-30">0-30</option>
         <option value="30-60">30-60</option>
         <option value="60-99999">60-99999</option>
    </select>
</form>

My Controller:

public static function getCalls($id)
    {
        if ($_GET['datum'] == "60-99999") {
            $sDates = explode("-", $_GET);
            $aCalls = // query

        } elseif ($_GET['datum'] == "30-60") {
            $sDates = explode("-", $_GET);
            $aCalls = // query

        } else {
            $aCalls = // query
        }

        return $aCalls;
    }
}

As an example, once submitted my $_GET holds:

array(1) { ["datum"]=> string(8) "60-99999" }

With this value I'd want the

if ($_GET == "60-99999")

query to show.