Calculator beginner issue

Hello guys, I am sorry to disturb you. I followed a tutorial and i am a newbe to php but want to learn. My calculator doesn't work, i dont see what is wrong. Please can someone explain what the issue is or give me a hint. Thank you in advance. Html Code: <form method="post" action="calc.php"> <input type="text" name="nummer1" placeholder="nummer1" /> <label for="kies">kies</label> <select name="reken" id="kies"> <option>selecteer</option> <option value ="plus">+</option> <option value ="min">-</option> <option value ="delen">÷</option> <option value ="maal">x</option> </select> <input type="number" name="nummer2" placeholder="nummer2" /> <input type="submit" value="calculeer" /> </form> php code: function bereken () { $nummer1 = $_POST['nummer1']; $nummer2 = $_POST['nummer2']; switch ($_POST['reken']) { case "plus": $plus = $nummer1 + $nummer2; return $plus; break; case "min": $min = $nummer1 - $nummer2; return $min; break; case "delen": $deel = $nummer1 / $nummer2; return $deel; break; case "maal": $maal = $nummer1 * $nummer2; return $maal; break; default: echo "bereken op een andere maniet"; } } echo "het resultaat is :". bereken(); ?> <br> <a href="index.html">terug</a>