Can we do this in PHP?

Hello Sir/Madam,

One thing doesn’t works for me in PHP about printing the full name using the class method User.

I did this

$record = User::find_by_id(1);
$user = new User();
$user->id = $record['id'];
$user->username = $record['username'];
$user->password = $record['password'];
$user->first_name = $record['first_name'];
$user->last_name = $record['last_name'];

echo 'First name is :' . $user->first_name;
echo '<br />Last name is :' . $user->last_name;
echo '<br />Full name is :' . $user->full_name();

I got the output as following:

First name is :Mohan
Last name is :Sinfh
Full name is :Empty

Full name is empty.
How it could be possible?

the method in User class I wrote is,

class User {

public $id;
public $username;
public $password;
public $first_name;
public $last_name;
	
public function full_name(){
if(isset($this->$first_name) && isset($this->last_name)) {
return $this->$first_name . ' ' . $this->$last_name;
}
else {return "Empty";}
}
}

Thanks.

if(isset($this->$first_name) && isset($this->last_name)) {

Ooops !
Got it…
Thank you sir.