PHP - NOTIFICACIÓN PHP

a=(isset($_GET['a'])?$_GET['a']:"");

<fieldset>

    <form autocomplete="off" action="?" method="get">

        <label>Numero 1:</label><input type="text" name="a"><br>

        <label>Numero 2:</label><input type="text" name="b"><br>

        <input type="submit" value="Sumar" name="enviar">

    </form>

    <br>

    <?php

        $a=(isset($_GET['a'])?$_GET['a']:"");

        $b=(isset($_GET['a'])?$_GET['b']:"");

        echo $a ."<br>";

        echo $b ."<br>";

        echo $a." + ".$b ." = " . ($a + $b);

    ?>

</fieldset>

<form action="?" method="post">

    <label>Widht:</label><input type="text" name="ancho"><br>

    <label>Height:</label><input type="text" name="alto"><br>

    <label>Color:</label><input type="color" name="color"><br>

    <input type="submit" value="Sumar">

</form>

<br>

<?php

	$ancho = $_POST['ancho'] . "px";

	$alto = $_POST['alto'] . "px";

	$color = $_POST['color'];

	echo $ancho . "<br>";

	echo $alto . "<br>";

	echo $color . "<br>";

?>

<style>

	div{

	width:<?php echo $ancho; ?>;

	height:<?php echo $alto; ?>;

	background:<?php echo $color; ?>;

	}

</style>

    <div></div>