if(isset($_POST['yech'])){

    $a = $_POST['son1'];

    $b = $_POST['son2'];

    $c = $a + $b;

    echo'<h2>'. $a.' + '.$b.' = '.$c.'</h2>';

}





echo'<form name="form" action="" method="POST">

        Birinchi sonni kiriting:<br>

        <input type="number" name="son1" required><br>

        Ikkinchi sonni kiriting:<br>

        <input type="number" name="son2" required><br>

        <input name="yech" type="submit" value="Yechimni ko\'rish">

    </form>';