Untitled

<html>
<head>
<title> Input page </title>
</head>
<body>
<?php
if($_POST) {

    if((!empty($_POST['address') && (!empty($_POST['rent']) && (!empty($_POST['bedrooms'])) {
        $username = "lettings";
        $password = "glass bottle picnic";
        $database = "lettings";

        $mysqli = mysqli_connect("localhost", $username, $password, $database);

        $address = mysqli_real_escape_string($mysqli, $_POST['address']);
        $rent = mysqli_real_escape_string($mysqli, $_POST['rent']);
        $bedrooms = mysqli_real_escape_string($mysqli, $_POST['bedrooms']);

        $query = mysqli_query($mysqli, INSERT INTO contacts VALUES ('', '".$address."', '".$rent."', '".$bedrooms."'));
        mysqli_close($mysqli);
    } else {
        echo "You left a field blank";
    }

} else {
    //what to do in the event no post has been recieved at all, could be a blank submission, or could be the first loading of the page
}

?>

your html input stuff could go here, because it's not enclosed in either branch of the if above, it'll be displayed whether we made a post or not

<form method="post" action=""> <!-- the action="" bit tells the browser to just post back to this page
<input type="text" name="address" id="address" />
<input type="submit" name="submit" value="Insert record" />
</form>
</body>
</html>