Subverter

index.html

Give the "Text Input" a name not an ID.

       <!-- This section is for: Take a quick Godzilla survey /-->
        <div id="survey-div">
            <form id="survey-form" class="MainPgTxtBckgColor" action = "GetFormData.php" method = "post">
                <fieldset id="survey-fieldset">
                    <legend>Take one of our quick surveys.</legend>
                    <p>
                        <label>1. Who is your <strong>favorite monster</strong> character (Godzilla related or not):</label>
                        <input name="favmonster" type = "text" value = "" />
                    </p>
                    <p>
                        <label>Press 'Submit' when you are done.</label>
                        <input type="submit" value="Submit" name="submit_button">
                    </p>
                </fieldset>
            </form>
        </div>

GetFormData.php
 Watch here how to print code output

 <?php
    // define variables and set to empty values
    $favmonsterErr = "";
    $favmonster = "";

    // verify that field is not empty
    if ($_SERVER['REQUEST_METHOD'] == 'POST') {
        if (empty($_POST['favmonster'])) {
            echo "Favorite Monster is required";
            echo '<input type="button" style="display: block" value="Back" onclick="window.history.back()" />';
        } else {
            $favmonster = test_input($_POST['favmonster']);
            echo "You Favorite Monster is: " .$favmonster;
            echo '<input type="button" style="display: block" value="Back" onclick="window.history.back()" />';
        }
    }

    function test_input($data) {
        $data = trim($data);
        $data = stripslashes($data);
        $data = htmlspecialchars($data);
        if (!preg_match('/^[a-zA-Z ]*$/',$data)) {
            $data = "Only letters and white space allowed";
        }
        return $data;
    }
    ?> 

Quote

$to = "xxxxxxxxxx@att.txt.net";

Seems you are missing a double quotation there