After the content of the body of the message, add this:
"FROM: enquriy@blahblahblah.co.uk"
So your code becomes:
<?php
$where_form_is="http://".$_SERVER['SERVER_NAME'].strrev(strstr(strrev($_SERVER['PHP_SELF']),"/"));
session_start();
if( ($_SESSION['security_code']==$_POST['security_code']) && (!empty($_POST['security_code'])) ) {
// Checkbox handling
$field_19_opts = $_POST['field_19'][0].",". $_POST['field_19'][1];
// Checkbox handling
$field_20_opts = $_POST['field_20'][0].",". $_POST['field_20'][1];
// Checkbox handling
$field_21_opts = $_POST['field_21'][0].",". $_POST['field_21'][1];
mail("enquiries@blahblah.co.uk","RESERVATION for " . $_POST['field_15'] . " for " . $_POST['field_16'] . " ","Reservation Information:
***********************************
" . $_POST['field_1'] . "
" . $_POST['field_2'] . "
" . $_POST['field_3'] . "
***********************************
" . $_POST['field_4'] . "
" . $_POST['field_5'] . "
" . $_POST['field_6'] . "
" . $_POST['field_7'] . "
" . $_POST['field_8'] . "
" . $_POST['field_9'] . "
" . $_POST['field_10'] . "
************************************
" . $_POST['field_11'] . "
" . $_POST['field_13'] . "
" . $_POST['field_14'] . "
************************************
" . $_POST['field_15'] . "
" . $_POST['field_16'] . "
************************************
Party Names
" . $_POST['field_17'] . "
************************************
Found Us By " . $_POST['field_18'] . "
************************************
Extras
$field_19_opts & $field_20_opts
************************************
Notes
" . $_POST['field_22'] . "
************************************
Discount Codes
" . $_POST['field_23'] . "
************************************
", "FROM: enquiry@blahblahblah.co.uk");
header('Location: ../html/submitok.html');
}
else
{echo "Invalid Security Code entered please click BACK and try again.";}
?>
The [man]mail/man function allows you to set your own headers. So you can set the DATE or REPLY-TO or any other typical header. Have a quick look at the documentation for some more information.