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>';