<form action="" method="post">
    <input type="text" id="address" name="address" placeholder="Enter your zip code" style="width:250px;" />
    <input type="submit" id="submit" value="Submit" />
</form>

<?php
    $textboxValue = $_POST['address'];
    echo $textboxValue;
?>

<?php
    if(isset($_POST['submit'])){
        $textboxValue = $_POST['address'];
        echo $textboxValue;
    }
?>

<input type="submit" name="submit" id="submit" value="Submit" />