app\models\Invoice::getEntityType PHP Method

    public function getEntityType()
    {
        return $this->isType(INVOICE_TYPE_QUOTE) ? ENTITY_QUOTE : ENTITY_INVOICE;
    }

 public function sendInvoice(Invoice $invoice, $reminder = false)
 {
     $invoice->load('invitations', 'client.language', 'account');
     $entityType = $invoice->getEntityType();
     $client = $invoice->client;
     $account = $invoice->account;
     if ($invoice->trashed() || $client->trashed()) {
         return false;
     }
     $account->loadLocalizationSettings($client);
     if ($account->pdf_email_attachment) {
         $invoice->updateCachedPDF();
     }
     $view = 'invoice';
     $accountName = $invoice->account->getDisplayName();
     $emailTemplate = $invoice->account->getEmailTemplate($reminder ?: $entityType);
     $emailSubject = $invoice->account->getEmailSubject($reminder ?: $entityType);
     $this->initClosure($invoice);
     $response = false;
     $sent = false;
     foreach ($invoice->invitations as $invitation) {
         if (Auth::check()) {
             $user = Auth::user();
         } else {
             $user = $invitation->user;
             if ($invitation->user->trashed()) {
                 $user = $account->users()->orderBy('id')->first();
             }
         }
         if (!$user->email || !$user->confirmed) {
             continue;
         }
         if (!$invitation->contact->email || $invitation->contact->trashed()) {
             continue;
         }
         $invitation->sent_date = \Carbon::now()->toDateTimeString();
         $invitation->save();
         $variables = ['account' => $account, 'client' => $client, 'invitation' => $invitation, 'amount' => $invoice->getRequestedAmount()];
         $data['body'] = $this->processVariables($emailTemplate, $variables);
         $data['link'] = $invitation->getLink();
         $data['entityType'] = $entityType;
         $data['invoice_id'] = $invoice->id;
         $subject = $this->processVariables($emailSubject, $variables);
         $fromEmail = $user->email;
         $response = $this->sendTo($invitation->contact->email, $fromEmail, $accountName, $subject, $view, $data);
         if ($response === true) {
             $sent = true;
             Activity::emailInvoice($invitation);
         }
     }
     if ($sent === true) {
         if (!$invoice->isSent()) {
             $invoice->invoice_status_id = INVOICE_STATUS_SENT;
             $invoice->save();
         }
         $account->loadLocalizationSettings();
         Event::fire(new InvoiceSent($invoice));
     }
     return $response ?: trans('texts.email_error');
 }