<?php

echo '<form action="index.php" method="get">';

if ($_GET['a']) {
    echo $_GET['a'];
}else{
    echo 'a <input type="number" required name="a" alt="0">  ';
}


if (isset($_GET['method'])) {
    echo $_GET['method'];
}else{
    echo '  <select name="method">'.'<option>Выберите действие</option>'.'<option selected value="+">+</option>'.'<option value="-">-</option>'.'<option value="*">*</option>'.'<option value="/">/</option>'.'</select>  ';

}

if (isset($_GET['b'])) {
    echo $_GET['b'];
}else{
    echo 'b <input type="number" required name="b" alt="0">  ';
}

if (isset($_GET['a']) && isset($_GET['method']) && isset($_GET['b'])) {
    if ($_GET['method'] == '+') {
        $result = $_GET['a'] + $_GET['b'];
        echo '= ' . $result;
    } else if ($_GET['method'] == '-') {
        $result = $_GET['a'] - $_GET['b'];
        echo '= ' . $result;

    } else if ($_GET['method'] == '*') {
        $result = $_GET['a'] * $_GET['b'];
        echo '= ' . $result;

    }else if ($_GET['method'] == '/') {
        $result = $_GET['a'] / $_GET['b'];
        echo '= ' . $result;

    }
} else {
    echo '   =   ';
    echo '    <input type="submit">';
}



echo '</form >';

?>