<?php
if ( !empty($_GET["v"]) ) {
    $valor = $_GET["v"];
    $rq = sqrt($valor);
    echo "Raiz quadrada de $valor = " .number_format($rq,2);
}
?>

<div>
    <form method="get" action="ModeloPHP.php">
        VALOR: <input type="number" name="v"/>
        <input type="submit" value="Calcular Raiz"/>

    </form>