You're saying

$hideval = $_GET['q'];

and

<form id="custform" method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">

Notice that the HTTP query string will not be sent, so when you click continue, $_GET['q'] will be not set.

Try to write it as follows:

<form id="custform" method="post" action="<?php echo $_SERVER['PHP_SELF']."?".http_build_query($_GET); ?>">

Also, it is in my opinion highly recommended to set the error reporting level to the most strict mode, to see all errors (assuming that you're not testing your website 'online', enabling the world to see while you're still testing):

error_reporting(-1); // Display *ALL* PHP errors, enabling you to see every error PHP will emit.

EDIT:

For me, this is working:

<?php

//require_once  ("Includes/connectDB.php");
session_start();
$pn = $_GET['q'];
$hideval = $pn;  //values are as expected here...(the calling form has checks in place to ensure valid number entered

$query = "SELECT * FROM customer WHERE Phone='$pn'";
$result = mysql_query($query);

if (!$result)
    echo ("Database access failed: " . mysql_error());
$rows = mysql_num_rows($result);

$rowinfo = mysql_fetch_array($result);

$cphone = $pn;
if ($rows > 0) {
    $clast = $rowinfo['LastName'];
    $cfirst = $rowinfo['FirstName'];
    $caddr1 = $rowinfo['Address1'];
    $caddr2 = $rowinfo['Address2'];
    $cpostal = $rowinfo['Postal'];
    $ccity = $rowinfo['City'];
    $cprov = $rowinfo['Province'];
    $cpay = $rowinfo['PayType'];
    $corder = $rowinfo['CustOrder'];
}
else {
    $clast = '';
    $cfirst = '';
    $caddr1 = '';
    $caddr2 = '';
    $cpostal = '';
    $ccity = '';
    $cprov = 'ON';
    $cpay = '';
    $corder = '';
}

function AddRecord() {
    $clast = $_POST['CustLastName'];
    $cfirst = $_POST['CustFirstName'];
    $cphone = $_POST['CustPhone'];
    $caddr1 = $_POST['CustAddress1'];
    $caddr2 = $_POST['CustAddress2'];
    $ccity = $_POST['CustCity'];
    $cpostal = $_POST['CustPostal'];
    $cpay = $_POST['PayType'];
    $corder = $_POST['CustOrder'];

    if (!get_magic_quotes_gpc()) {
        $cphone = addslashes($cphone);
        $clast = addslashes($_POST['CustLastName']);
        $cfirst = addslashes($_POST['CustFirstName']);
        $caddr1 = addslashes($_POST['CustAddress1']);
        $caddr2 = addslashes($_POST['CustAddress2']);
        $ccity = addcslashes($_POST['CustCity']);
        $cpostal = addcslashes($_POST['CustPostal']);
        $corder = addcslashes($_POST['CustOrder']);
    } else {
        $clast = ($_POST['CustLastName']);
        $cfirst = ($_POST['CustFirstName']);
        $caddr1 = ($_POST['CustAddress1']);
        $caddr2 = ($_POST['CustAddress2']);
        $ccity = ($_POST['CustCity']);
        $cpostal = ($_POST['CustPostal']);
        $corder = $_POST['CustOrder'];
    }

    $query = "INSERT INTO customer (Phone, LastName, FirstName, Address1, Address2, City, Province, Postal, PayType, CustOrder) VALUES ('$cphone','$clast','$cfirst', '$caddr1', '$caddr2', '$City', '$Province', '$Postal', '$cpay', '$corder')";
    $result = mysql_query($query);
    if (!$result)
        die("Database access failed: " . mysql_error());
}

function EditRecord() {
    echo ('Im going to try an edit');
    die;
    $clast = $_POST['CustLastName'];
    $cphone = $_POST['CustPhone'];
    $cfirst = $_POST['CustFirstName'];
    $caddr1 = $_POST['CustAddress1'];
    $caddr2 = $_POST['CustAddress2'];
    $ccity = $_POST['CustCity'];
    $cpostal = $_POST['CustPostal'];
    $cpay = $_POST['PayType'];
    $corder = $_POST['CustOrder'];

    if (!get_magic_quotes_gpc()) {
        $clast = addslashes($_POST['CustLastName']);
        $cfirst = addslashes($_POST['CustFirstName']);
        $caddr1 = addslashes($_POST['CustAddress1']);
        $caddr2 = addslashes($_POST['CustAddress2']);
        $ccity = addcslashes($_POST['CustCity']);
        $cpostal = addcslashes($_POST['CustPostal']);
        $corder = addslashes($_POST['CustOrder']);
    } else {
        $clast = ($_POST['CustLastName']);
        $cfirst = ($_POST['CustFirstName']);
        $caddr1 = ($_POST['CustAddress1']);
        $caddr2 = ($_POST['CustAddress2']);
        $ccity = ($_POST['CustCity']);
        $cpostal = ($_POST['CustPostal']);
        $corder = $_POST['CustOrder'];
    }

    $query = "UPDATE customer SET Phone='$cphone', LastName='$clast', FirstName='$cfirst', Address1='$caddr1', Address2='$caddr2', City='$ccity', Province='$cprov', Postal='$cpostal', PayType='$cpay', CustOrder='$corder' WHERE Phone='$cphone'";
    $result = mysql_query($query);

    if (!$result)
        echo ("Database access failed: " . mysql_error());
}

?>

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
        <link rel="stylesheet" type="text/css" href="styles/beer_delivery.css" />
    </head>
    <body>
        <p></p>
        <br><br>
                <div id="mainPicture">
                    <div class="picture">

                    </div>
                </div>
                <div class="contentBox">
                    <div class="innerBox">
                        <h1>Customer Information</h1>
                        <div class="contentText">
                            <p></p>
                            <div id="main">
                                <form action="<?php echo $_SERVER['PHP_SELF'] . "?" . http_build_query($_GET); ?>" method="post">
                                    <fieldset>
                                        <legend>Customer</legend>
                                        <label for="CustPhone">Phone:</label>

                                        <input type="text" name="CustPhone" id="CustPhone" tabindex="2" value="<?php echo $cphone; ?>">
                                            <br>
                                                <div id="divider"></div>

                                                <p>&nbsp;</p>
                                                <label for="CustLastName">Last Name:</label>
                                                <input type="text" name="CustLastName" id="CustLastName" tabindex="2" value="<?php echo $clast; ?>">
                                                    &nbsp;&nbsp;&nbsp;&nbsp;
                                                    <label for="CustFirstName">First Name:</label>
                                                    <input type="text" name="CustFirstName" id="CustFirstName" tabindex="3" value="<?php echo $cfirst; ?>">

                                                        <p>&nbsp;</p>

                                                        <label for="CustAddress1">Address 1:</label>
                                                        <input type="text" name="CustAddress1" id="CustAddress1" tabindex="4" value="<?php echo $caddr1; ?>">

                                                            &nbsp;&nbsp;&nbsp;&nbsp;
                                                            <label for="CustAddress2">Address2:</label>
                                                            <input type="text" name="CustAddress2" id="CustAddress2" tabindex="5" value="<?php echo $caddr2; ?>">

                                                                <p>&nbsp;</p>

                                                                <label for="CustCity">City:</label>
                                                                <input type="text" name="CustCity" id="CustCity" tabindex="6" value="<?php echo $ccity; ?>">

                                                                    <p>&nbsp;</p>

                                                                    <label for="CustPostal">PostalCode</label>
                                                                    <input type="text" name="CustPostal" id="CustPostal" tabindex="7" value="<?php echo $cpostal; ?>">

                                                                        &nbsp;&nbsp;&nbsp;&nbsp;
                                                                        <label for="CustProvince">Province:</label>
                                                                        <input type="text" name="CustProvince" id="CustProvince" tabindex="8" value="<?php echo $cprov; ?>">

                                                                            <p>&nbsp;</p>
                                                                            <label for="CustOrder">Customer Order:</label>
                                                                            <textarea name="CustOrder" rows="4" cols="50" tabindex="9"><?php echo $corder; ?></textarea>
                                                                            <p>&nbsp;</p>

                                                                            <label for="PayType">Method of Payment:</label>
                                                                            <select name="PayType" id="PayType" tabindex="10">
                                                                                <option value="Credit Card" <?php if ($cpay == 'Credit Card') echo ' selected="selected"'; ?>>Credit Card</option>
                                                                                <option value="Debit" <?php if ($cpay == 'Debit') echo ' selected="selected"'; ?>>Debit</option>
                                                                                <option value="Cheque" <?php if ($cpay == 'Cheque') echo ' selected="selected"'; ?>>Cheque</option>
                                                                                <option value="Other"<?php if ($cpay == 'Other') echo ' selected="selected"'; ?>>Other</option>
                                                                            </select>

                                                                            <p>&nbsp;</p>
                                                                            &nbsp;&nbsp;&nbsp;&nbsp;

                                                                            <form id="custform" method="post" action="<?php echo $_SERVER['PHP_SELF'] . "?" . http_build_query($_GET); ?>">
                                                                                <input type="submit" name="Continue" id="Continue" value="Continue" tabindex="11">

                                                                            </form>
<?php

//have also used session vars here...but same problem
echo $_GET['q']; //this shows the phone number
if (isset($_POST['Continue'])) {
    //ultimately, the functions AddRecord and EditRecord will be called in here ... once I get the variable problem fixed
    echo 'after click: ' . $_GET['q'];  //this shows "after click: "
}

?>

                                <p>&nbsp;</p>
                            </fieldset>
                        </form>
                    </div>
                </div>
            </div>
        </div>

    </body>
</html>

When I type http://example.com/customer_info.php?q=4, example.com being the server where you're testing it, often localhost, into the address bar, the website does show right after the Continue button:

4

and when I click the button, it shows:

4after click: 4

For me, it's working. If it's for you still not working, then check the session settings. Maybe there's something (wrong) with it.

The reason is that you declare $_SESSION['hv'] = $hideval; on the second site of your form, too, which propably has a null value because the respective form element does not exist.

Instead, you would propably want to declare it like that:

if(!empty($_GET['q'])) {
    $_SESSION['hv'] = $_GET['q'];
}

On the next site the session variable won't be overwritten with null and you can access it as expected.

And that is what MildlyInteresting is saying. Try to replace

$hideval = $_GET['q']; 

with

if(!empty($_GET['q'])) {
    $hideval = $_GET['q'];
}

obviously,the $_POST['Continue'] is the problem,you can use var_dump to print the $_POST varible,to check the data from form.

var_dump($_POST);

if there is no $_POST['Continue'], surely you can not get the value of $hideval. hope useful to you!