echo '<form method="post" action="index.php">';
    echo '<input type="hidden" name="hoge" value="'.$_POST["hoge"].'">';
    echo '<button name="backbtn" type="submit">入力ページへ</button>';
echo '</form>';

if (isset($_POST["backbtn"])) {
    $hoge = $_POST["hoge"];
} else {
    $hoge = '';
}

<form method="post" action="confirm.php">
    <input type="text" name="hoge" value="<?=$hoge?>">
    <button type="submit">確認</button>
</form>