you can't "post to a txt file."
well, you can, but it would be pointless.
Post to a php script that will write to a txt file.

What format do you want the form info to be saved in?
Here's an example using JSON:

<?php
// check if a form was submitted
if( !empty( $_POST ) ){
    // convert form data to json format
    $json = json_encode( $_POST );
    // make sure there were no problems
    if( json_last_error() != JSON_ERROR_NONE ){
        exit;  // do your error handling here instead of exiting
    }
    // write to file
    //   note: _server_ path, NOT "web address (url)"!
    file_put_contents( '/server/path/to/json.txt',$json );
}

This example will actually save any and all info your form submits.

Depending on what you're doing with the info, you might need to take extra precautions (for example, strip html tags if you need the file to be accessible via the web).

Have you considered using a database? This is exactly the sort of thing they're for.

The. txt file approach will work also (and JSON is an excellent format for this purpose), but a DB is preferable.

ahh, but are you willing? (and it's nothing to be sorry about!) Does your hosting plan include a database?

If you'd rather use a .txt file, that's fine too. Here's a little more of an example based on your description:

<?php
// check that form was submitted
//   (you'll need to change these indices to match your form field names)
if( !empty( $_POST['firstname'] ) && !empty( $_POST['lastname'] ) ){
    // remove html tags from submission
    //   (since you don't want them)
    $firstname = strip_tags( $_POST['firstname'] );
    $lastname = strip_tags( $_POST['lastname'] );
    // create the date
    //   (you can change the format as desired)
    $date = date( 'Y-m-d' );
    // create an array that holds your info
    $record = array( $firstname,$lastname,$date );
    // save the record to your .txt file (I still recommend JSON)
    $json = json_encode( $record );
    $file = '/_server_/path/to/yourfile.txt';
    file_put_contents( $json,$file );
}

to retrieve the data later, you could use something like this:

<?php
// read the .txt file into an array
$file = '/_server_/path/to/yourfile.txt';
$jsonarray = file( $file,FILE_IGNORE_NEW_LINES,FILE_SKIP_EMPTY_LINES );
// convert each array element (each record) back into an array
foreach( $jsonarray as $json ){
    $records[] = json_decode( $json,TRUE );
}
// loop through the records and write your html to display
//   (I'm guessing you're putting this data into a table)
foreach( $records as $record ){
    // extract each variables
    list( $firstname,$lastname,$date ) = $record;
    // EDIT: added htmlentities() for safe output.
    $firstname = htmlentities( $firstname );
    $lastname = htmlentities( $lastname );
    // make an array of table rows
    $trs[] = "
    <tr>
        <td>$firstname</td>
        <td>$lastname</td>
        <td>$date</td>
    </tr>";
    // cleanup
    unset( $firstname,$lastname,$date );
}
// prepare the table
$table = '
<table><tbody>
    <tr>
        <th>First Name</th>
        <th>Last Name</th>
        <th>Date</th>
    </tr>'.
    implode( '',$trs ).'
</tbody></table>';
// when you're ready, print it to your page
print $table;

of course, this will vary depending on how you want to display the data, but that's the basic idea.

You could probably modify the way you store the data to match the way your current script reads the .txt file, but that might be more trouble than it's worth ...it would depend. The approach in my example is a good one, however.

Using a database instead would:
-- be more robust
-- require less code to prepare/store/retrieve/decode the data
-- allow you to search for specific records efficiently if the need ever arose

?, lmk

When you have a chance, read up on some sql tutorials. You might use a DB structure like this:

CREATE TABLE `baby_registry`(
    `id` INT( 11 ) NOT NULL AUTO_INCREMENT
   ,`firstname` VARCHAR( 255 ) NOT NULL
   ,`lastname` VARCHAR( 255 ) NOT NULL
   ,`date` DATETIME NOT NULL
   ,PRIMARY KEY( `id` )
) ENGINE=MyISAM  DEFALUT CHARSET=utf8 ;

storing a record (mysqli example):

<?php
$DB = new mysqli();
$firstname = $DB->real_escape_string( $firstname );
$lastname = $DB->real_escape_string( $lastname );
$SQL = "INSERT INTO `baby_registry`( `firstname`,`lastname`,`date` )
VALUES( '$firstname','$lastname',UTC_TIMESTAMP() )";
$DB->query( $SQL );

retrieving a record (mysqli example):

<?php
// first, tell mysql what timezone you're using
//   for timezone names, see http://php.net/manual/en/timezones.php
$DB->query( "SET time_zone = 'America/Los_Angeles'" );
// retrieve data
$SQL = "SELECT `firstname`,`lastname`,`date` ) 
FROM `baby_registry` 
ORDER BY `date`";
$result = $DB->query( $SQL );
if( $result ){
    while( $r = $result->fetch_row() ){
        list( $firstname,$lastname,$date ) = $r;
        // continue normally from here
    }
}

On the PHP end, avoid the mysql extension: learn mysqli (or PDO) instead.

your script uses several ereg_* functions, which are depreciated in PHP since version 5.3, and was highly discouraged for quite some time before that.

Additionally, using die() as an error handler is not optimal, because it leaves the user with a broken page.

You might also look at fgetcsv(), which cleanly handles writing files in comma-separated-value format.

Not giving up on you. If you don't have the time to work on this, you'll need to either take your time with it, or consider hiring someone. I do freelance work like this, and I'm still willing to help if you want to try it yourself based on the examples I posted. Good luck

It's a possible solution. I think it's a little "overkill," and probably would not be less work in the short term.

But if you moved the entire site to WP (not just your "registry" page), then it could be very beneficial, especially in the long run.