Why does Laravel API Resource data come this way?

I’m writing an API Resource. I think it’s about OOP. In the first code, the fullName field is empty, more precisely, only last_name comes. But in the second code it works exactly as I wanted it. What is the reason of this?

FİRST

 public function toArray($request)
    {
        return [
            'user_id'       => $this->id,
            'email'         => $this->email,
            'description'   => $this->description,
            'first_name'    => $this->first_name,
            'last_name'     => $this->last_name,
            'full_name'     => $this->firstname . ' ' . $this->last_name,
        ];
    }

SECOND

public function toArray($request)
{
    $firstName = $this->first_name;
    $lastName  = $this->last_name;
    $fullName  = $firstName . ' ' . $lastName;
    return [
        'user_id'       => $this->id,
        'email'         => $this->email,
        'description'   => $this->description,
        'first_name'    => $this->first_name,
        'last_name'     => $this->last_name,
        'full_name'     => $fullName,
    ];
}

Answer

You have $this->firstname in the first one. It should be $this->first_name. So it should look like this:

'full_name' => $this->first_name . ' ' . $this->last_name,