Just tried to make something of your code. But there are a few things you really need to take a look at:

1. You are missing some closing tags in your html, for example you are missing a </div> & </form> tag.
2. You can put all your php code at the top of your page. It's a best practice to seperate your logic from your view.
3. Proper php opening and closing tags looks like the following : <?php ... ?> not <= or <?

4. A proper PDO connection looks like this

   <?php try { $db = new PDO("pgsql:dbname=pdo;host=localhost", "username", "password" ); echo "PDO connection object created"; } catch(PDOException $e){ echo $e->getMessage(); } ?>

5. If you want to concatenate a ul to output your errors you will have to do that the as follow:

   <?php str .= '<li></li>'; str .= '</ul>'; ?>

The foundation of the errors you are getting are caused by points like the above. Goodluck and ask if you have any more questions.

You could make two forms with 2 different actions

<form action="login.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="submit" value="Login">
</form>

<br />

<form action="register.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="submit" value="Register">
</form>

Or do this

<form action="doStuff.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="hidden" name="action" value="login">
    <input type="submit" value="Login">
</form>

<br />

<form action="doStuff.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="hidden" name="action" value="register">
    <input type="submit" value="Register">
</form>

Then you PHP file would work as a switch($_POST['action']) ... furthermore, they can't click on both links at the same time or make a simultaneous request, each submit is a separate request.

Your PHP would then go on with the switch logic or have different php files doing a login procedure then a registration procedure

You can tell the form to submit to the PHP's self, then check the $_POST variables for form processing. This method is very good for error checking as you can set an error and then have the form reload with any information the user's previously submitted still in tact (i.e. they don't lose their submission).

When the "submit" button is clicked, it will POST the information to the same page, running the PHP code at the top. If an error occurs (based on your checks), the form will reload for the user with the errors displayed and any information the user supplied still in the fields. If an error doesn't occur, you will display a confirmation page instead of the form.

<?php
//Form submitted
if(isset($_POST['submit'])) {
  //Error checking
  if(!$_POST['yourname']) {
    $error['yourname'] = "<p>Please supply your name.</p>\n";
  }
  if(!$_POST['address']) {
    $error['address'] = "<p>Please supply your address.</p>\n";
  }

  //No errors, process
  if(!is_array($error)) {
    //Process your form

    //Display confirmation page
    echo "<p>Thank you for your submission.</p>\n";

    //Require or include any page footer you might have
    //here as well so the style of your page isn't broken.
    //Then exit the script.
    exit;
  }
}
?>

<form method="post" action="<?=$_SERVER['PHP_SELF']?>">
  <?=$error['yourname']?>
  <p><label for="yourname">Your Name:</label><input type="text" id="yourname" name="yourname" value="<?=($_POST['yourname'] ? htmlentities($_POST['yourname']) : '')?>" /></p>
  <?=$error['address']?>
  <p><label for="address">Your Address:</label><input type="text" id="address" name="address" value="<?=($_POST['address'] ? htmlentities($_POST['address']) : '')?>" /></p>
  <p><input type="submit" name="submit" value="Submit" /></p>
</form>

You can check in the same file if the submit button is pressed. (http://pastebin.com/8FC1fFaf)

if (isset($_POST['submit'])) 
{ 
    // Process Form
}
else
{
    // Show Form
}

Regarding form checking, you can save the user input, and if one of their data is unvalid you can echo the old data back and show the form again.

edit: As PeeHaa stated, you need to leave the action blank though ;)

Do you mean something like this? Each submit button has its own validation.

<?php
    if (isset($_POST['submit_data'])) {
        //first form
    }
    
    if (isset($_POST['edit_data'])) {
        //second form
    }
?>


<form method="post" action="<?php $_SERVER["PHP_SELF"]; ?>">
    <label for='name'>Naam:</label><br>
    <input type="text" id="name" name="name" placeholder = "John Doe" required><br>
    <input type="submit" name="submit_data" value="Save"><br>
</form>

<form method="post" action="<?php $_SERVER["PHP_SELF"]; ?>">
    <label for='name'>Naam:</label><br>
    <input type="text" id="name" name="name" placeholder = "John Doe" required><br>
    <input type="submit" name="edit_data" value="Edit"><br>
</form>

This should do what you want. It loads previous posts from posts.txt, adds the current one, displays the posts and saves it. You'll need to make sure posts.txt exists and has correct permissions.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml">
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 <title>Untitled Document</title>
 </head>

<body>
 <form id="form1" name="form1" method="post" action="">
 <label>Please type in a message
 <input type="text" name="msg" id="msg" />
 </label>
 <label>and your name
 <input type="text" name="name" id="name" />
 </label>

 <p>
 <label>Submit
 <input type="submit" name="submit" id="submit" value="Submit" />
 </label>
 </p>
 </form>

 <?php
    $msg = $_POST["msg"];
    $name = $_POST["name"];
    $posts = file_get_contents("posts.txt");
    $posts = "$msg - $name\n" . $posts;
    file_put_contents("posts.txt", $posts);
    echo $posts;
 ?>

</body>
 </html>

In order to stay on the same page on submit you can leave action empty (action="") into the form tag, or leave it out altogether.

For the message, create a variable ($message = "Success! You entered: ".$input;") and then echo the variable at the place in the page where you want the message to appear with <?php echo $message; ?>.

Like this:

Copy<?php
$message = "";
if(isset($_POST['SubmitButton'])){ //check if form was submitted
  $input = $_POST['inputText']; //get input text
  $message = "Success! You entered: ".$input;
}    
?>

<html>
<body>    
<form action="" method="post">
<?php echo $message; ?>
  <input type="text" name="inputText"/>
  <input type="submit" name="SubmitButton"/>
</form>    
</body>
</html>

This is what hidden inputs are for. Add one for your id:

<input type="hidden" name="id[]" value="<?=$rows['id']?>" />

Also, you don't need to save the count. You can count the number of items in an array using count($id).

Finally, sanitize your inputs. Never put user submitted data straight into a query. Use intval for integers and mysql_real_escape_string for strings, or use prepared statements.