noob: display err_msg on page

I have a form that populates a database, and I'm having trouble with the error handling. I want the errors to show in a small pop up window on the same pg.

    <form id="form1" name="form1" method="post" action="mailform.php" onsubmit="MM_validateForm('Name','','R','Business Name','','R','Email Address','','R','How selling product','','R','Where did you hear about us','','R');return document.MM_returnValue">
    <div style="color:#FF0000; text-align:center;"><?php if(!empty($_GET['err_msg'])){echo $_GET['err_msg'];} ?></div>
        <fieldset>
        <legend>Contact form</legend>
            <p class="first">
                <label for开发者_如何学Go="name">First Name</label>
                <input type="text" name="First Name" id="first_name" size="30" value="<?=htmlentities($profiledata['First_Name'])?>" />
            </p>

        <p class="first">
                <label for="name">Last Name</label>
                <input type="text" name="Last Name" id="last_name" size="30" value="<?=htmlentities($profiledata['Last_Name'])?>" />
            </p>

            <p>
                <label for="name">Phone Number</label>
                <input type="text" name="Phone Number" id="phone number" size="30" value="<?=$profiledata['Phone_Number']?>" />
            </p>
                            <p>
                <label for="email">Business Name</label>
                <input type="text" name="Business Name" id="business name" size="30" value="<?=htmlentities($profiledata['Business_Name'])?>" />
            </p>
                            <p>
                <label for="email">Web Address</label>
                <input type="text" name="Web Address" id="web address" size="30" value="<?=$profiledata['Web_Address']?>" />
            </p>
            <p>
                <label for="email">Email</label>
                <input type="text" name="Email Address" id="email address" size="30" value="<?=$profiledata['Email_Address']?>" />
            </p>
        <p>
            <img src="CaptchaSecurityImages.php?width=100&height=40&characters=5" /><br />
                <label for="security_code">Security Code: </label><input id="security_code" name="security_code" type="text" />
        </p>                
        </fieldset>
        <fieldset>                                                                          
            <p>
                <label for="message">Describe how you plan on selling this product</label>
                <textarea name="How selling product" id="How selling product" cols="30" rows="4"><?=htmlentities($profiledata['How_selling_product'])?></textarea>
            </p>    
            <p>
                <label for="message">Where did you hear about us?</label>
                <textarea name="Where did you hear about us" id="Where did you hear about us" cols="30" rows="4"><?=htmlentities($profiledata['Where_did_you_hear_about_us'])?></textarea>
            </p>                    
        </fieldset>
        <p class="submit"><button type="submit">Send</button></p>

        <input name="mailform_address" type="hidden" value="email@address.com" /> 
  </form>

Error handling:

function valid_email($str)
{
    return ( ! preg_match("/^([a-z0-9\+_\-]+)(\.[a-z0-9\+_\-]+)*@([a-z0-9\-]+\.)+[a-z]{2,6}$/ix", $str)) ? FALSE : TRUE;
}

function is_url($str)
{
    return ( ! preg_match("/^((www)\.)?((\w+|[\d]?+)+(\.|\-)(\w+[\d]?+))+(\w)$/", $str)) ? FALSE : TRUE;
}

function valid_phone($str)
{
    $Num = $str;
    $Num = ereg_replace("([     ]+)","",$Num);;
    $Num = eregi_replace("(\(|\)|\-|\+)","",$Num);
    if(!is_numeric($Num))
    {       
        return FALSE;
    }   
    else
        return TRUE;
}

$form_field =   array(  
                            'First_Name'                    =>  '',
                            'Last_Name'                     =>  '',
                            'Business_Name'                 =>  '',
                            'Phone_Number'                  =>  '',
                            'Web_Address'                   =>  '',
                            'Email_Address'                 =>  '',
                            'How_selling_product'           =>  '',
                            'Where_did_you_hear_about_us'   =>  '',
                        );
foreach($form_field as $key => $value){$profiledata[$key]=trim($_POST[$key]);}

$_SESSION['profiledata']    =   $profiledata;

$emailto = NULL;
$emailmessage = "Dealer Contact Form\n";
$emailsubject = "Dealer Contact Form";

if(!empty($_POST)){

    //echo "<pre>";print_r($_POST);die;

    if( $_SESSION['security_code'] != $_POST['security_code'] || empty($_SESSION['security_code'] ) ) {

        // Insert your code for showing an error message here
        $err_msg    = 'Sorry, you have provided an invalid security code';
        header("Location: reseller.php?err_msg=".urlencode($err_msg));
        return;

    } else {
        unset($_SESSION['security_code']);  
    }

    $fname      =   html_entity_decode(trim($_POST['First_Name']));
    $lname      =   html_entity_decode(trim($_POST['Last_Name']));
    $company    =   html_entity_decode(trim($_POST['Business_Name']));
    $phone      =   html_entity_decode(trim($_POST['Phone_Number']));
    $website    =   html_entity_decode(trim($_POST['Web_Address']));
    $email      =   html_entity_decode(trim($_POST['Email_Address']));
    $notes      =   "Lead Source: ".html_entity_decode(trim($_POST['Where_did_you_hear_about_us']))."\n";
    $notes      .=  "Selling Method: ".html_entity_decode(trim($_POST['How_selling_product']));

    if(!valid_phone($phone)){
        $err_msg    = 'Please enter valid Phone Number.';
        header("Location: reseller.php?err_msg=".urlencode($err_msg));
        return;
    }

    if(!is_url($website)){
        $err_msg    = 'Please enter valid Web Address.';
        header("Location: reseller.php?err_msg=".urlencode($err_msg));
        return;
    }

    if(!valid_email($email)){
        $err_msg    = 'Please enter valid Email.';
        header("Location: reseller.php?err_msg=".urlencode($err_msg));
        return;
    }

    if(!stristr($website,"http://") && !stristr($website,"https://") && $website){
        $website    =   "http://".$website;
    }

    $res    =   mysql_query("SELECT in_customer_id FROM tbl_customer WHERE st_company_name = '".addslashes($company)."'");
    if(mysql_num_rows($res)){   
        $err_msg    =   'Business Name already exists';
        header("Location: reseller.php?err_msg=".urlencode($err_msg));
        return;
    }

    $res    =   mysql_query("SELECT st_user_name,st_user_email_id FROM tbl_admin_user WHERE st_user_email_id='".addslashes($email)."' AND flg_is_delete=0");
    if(mysql_num_rows($res)){   
        $err_msg    =   'Email already exists';
        header("Location: reseller.php?err_msg=".urlencode($err_msg));
        return;
    }

I'd appreciate some help or at least a nudge in the right direction. Thanks!

Depending on what kind of 'popup' you want, you will probably want to implement some client side javascript that displays a nicely formatted message that makes use of the message that you send back from the server.

I'd look at using JQuery if it was me.

Is this okay?

<html>

<head>
<script>
function Start(page) {
OpenWin = this.open(page, "CtrlWindow", "toolbar=no,menubar=no,location=no,scrollbars=yes,resizable=yes,width=600,height=200,left="
+((window.screen.width-600)/2)+",top="+((window.screen.height-200)/3)+"" );
}
</script>
</head>

<body  onload="Start('child.htm')">
This is parent.
</body>
</html>