Simple question regarding isset

Okay so I have a self referencing form like so:

<form enctype="multipart/form-data" method="post" action=<?php echo $_SERVER['PHP_SELF']; ?>>

<p><label for="first_name">First Name:</label>
<input type="text" id="first_name"  value="<?php echo $first_name; ?>" /></p>
<p><label for="last_name">Last Name:</label>
<input type="text" id="last_name"  value="<?php echo $last_name; ?>" /></p>

<p><input name="submit" class = "submit" type="submit" value="Submit Information" /></p>
</form>

HTML:

I started running some error logs today and I found that everytime I would get an error like this:

Unknown error [#8]: Undefined variable: first_name
in /home/mysite.com/layout_includes/contact.php on line 7
Unknown error [#8]: Undefined variable: last_name
in /home/mysite.com/layout_includes/contact.php on line 9

So my question is would something like below be the actual "proper" way to code these variables, meaning we need to check if they exist before trying to echo them?

<form enctype="multipart/form-data" method="post" action=<?php echo $_SERVER['PHP_SELF']; ?>>

<p><label for="first_name">First Name:</label>
<input type="text" id="first_name"  value="<?php if(isset($first_name)) {echo $first_name;} ?>" /></p>
<p><label for="last_name">Last Name:</label>
<input type="text" id="last_name"  value="<?php if(isset($last_name)) {echo $last_name;} ?>" /></p>

<p><input name="submit" class = "submit" type="submit" value="Submit Information" /></p>
</form>

HTML:

I am not only looking for a debate on whether or not to use self referencing forms, if you will first be kind enough to answer my main question, then feel free to expand on other ideas!

 

I think you should use $_POST along with isset

value="<?php if(isset($_POST['first_name'])) echo $_POST['first_name']; ?>"

Do the same thing for other variables.

 

Sorry I left out some of my script:

If (isset($_POST['submit'])) {
	
	$validation = new Validate;
	
	
	$main = $_POST['main'];
	$contact = $_POST['contact'];
	$first_name = $_POST['first_name'];
	$last_name = $_POST['last_name'];
	$phone = $_POST['phone_number'];
	$email = $_POST['email'];
	$comment = $_POST['comment'];
	$email_fail = false;
	$phone_fail = false;
	

    if (!($validation->isEmail($email))){
		$email_fail = true;
	}
	if (!($validation->isPhone($phone))){
		$phone_fail = true;
	}
	if (!$phone_fail && !$email_fail){
		include ('utilities/email.php');	
	}


}

PHP:

Now, with that said, do I still need to be checking to see if the POST variables are set in the form include? As my code works, I suppose the POST variables are only set to the non global variables if they exist...so I suppose checking for POST is proper?

The whole point of this thread was to start a discussion so I better understand what the best way to work this is, so I appreciate the feedback, please keep it coming.