Printing user validation errors from .php file back onto a form

Question

Hey all. I have written a simple contact form with the extention .php. It includes the php validation and the form inputs on one page. If the input does not validate, a warning in red is printed at the top of the form after the user pushes submit. When I cut and paste the php validation code into a seperate file, the validation errors print on a blank screen. What I want is to have the same results with a seperate php file as I do when I include the php code on the form. (print errors from the seperate php validation file back onto the form).  Thanks for taking a look
<h1><span class="bodyheader1">Contact </span><span class="bodyheader2">Us</span></h1>
    <p class="subheading">Tell us what's on your mind:</p>
    <?php
    if(isset($_POST['send_comment']))
        //Validating the comment field
        //checking if a first_name field has been entered
        if($_POST['first_name'] != "")
            //Saitizing first_name with a filter
            $_POST['first_name'] = filter_var($_POST['first_name'], FILTER_SANITIZE_STRING);
            //Checking for no input
            if($_POST['first_name'] == "")
                //If no input, return error message
                $errors .='Please enter a valid first name.<br/>';
        else
            //Error message
            $errors .='Please enter your first name.<br/>';
        //Validating the last_name field
        //checking if last_name has been entered
        if($_POST['last_name'] != "")
            //Saitizing last_name with a filter
            $_POST['last_name'] = filter_var($_POST['last_name'], FILTER_SANITIZE_STRING);
            //Checking for no input
            if($_POST['last_name'] == "")
                //If no input, return error message
                $errors .='Please enter a valid last name.<br/>';
        else
            //Error message
            $errors .='Please enter your last name.<br/>';
        if($_POST['email'] != "")
            $email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);
            if (!filter_var($email, FILTER_VALIDATE_EMAIL))
                $errors .= "$email is not a valid email address.<br/>";
        else
            $errors .= 'Please enter your email address.<br/>';
        //Validating the comment field
        //checking if a comment has been entered
        if($_POST['comment'] != "")
            //Saitizing comment with a filter
            $_POST['comment'] = filter_var($_POST['comment'], FILTER_SANITIZE_STRING);
            //Checking for no input
            if($_POST['comment'] == "")
                //If no input, return error message
                $errors .='Please enter a comment.<br/>';
        else
            //Error message
            $errors .='Please enter a comment.<br/>';
        if(!$errors)
            $mail_to = '[email protected]';
            $subject = 'Contact form ' . $_POST['first_name'];
            $message = 'First Name: ' . $_POST['first_name'] . "\n";
            $message .= 'Last Name: ' . $_POST['last_name'] . "\n";
            $message .= 'Email: ' . $_POST['email'] . "\n";
            $message .= 'Comment: ' . $_POST['comment'] . "\n\n";
            mail($mail_to, $subject, $message);
            echo "<script>document.location.href='thankyou.html'</script>";
        else
            echo '<div style="color: red">' . $errors . '<br/></div>';
?>
*All fields are required<br />
<br />
    <form name="contact" method="post" action="contact.php">
        <p>First Name: <br />
          <input name="first_name" type="text" value="<?php echo $_POST['first_name']; ?>" size="30" maxlength="50" /><br />
          <br />
          Last Name: <br />
          <input name="last_name" type="text" value="<?php echo $_POST['last_name']; ?>" size="30" maxlength="50" /><br />
          <br />
          E-mail: <br />
          <input name="email" type="text" value="<?php echo $_POST['email']; ?>" size="40" maxlength="200" /><br />
          <br />
          Comment: <br />
          <textarea name="comment" rows="5" cols="50"><?php echo $_POST['comment'];?></textarea>
          <br />
         </p>
      <p><input type="submit" name="send_comment" />
      </p>
    </form>

Answer 1

You want the form to submit its results to a separate file which processes the data.  If this separate file finds errors, you want it to return control to the form page which would a) display the error messages, and b) populate the fields with the data entered?
If that's the case, then the processing file would have to send the errors and the data back to the form page either as a) URL variables, b) a cookie, or c) entries in a database.
Does that help?