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[PHP] 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 value ="plus">+</option>
        <option value ="min">-</option>
        <option value ="delen">÷</option>
        <option value ="maal">x</option>
    <input type="number" name="nummer2" placeholder="nummer2" />
    <input type="submit" value="calculeer" />


php code:

function bereken ()
    $nummer1 = $_POST['nummer1'];
    $nummer2 = $_POST['nummer2'];

    switch ($_POST['reken'])
        case "plus":
            $plus = $nummer1 + $nummer2;
            return $plus;

        case "min":
            $min = $nummer1 - $nummer2;
            return $min;

        case "delen":
            $deel = $nummer1 / $nummer2;
            return $deel;

        case "maal":
            $maal = $nummer1 * $nummer2;
            return $maal;
            echo "bereken op een andere maniet";

    echo "het resultaat is :". bereken();

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