app\models\Invoice::isSent PHP Method

    public function isSent()
    {
        return $this->invoice_status_id >= INVOICE_STATUS_SENT;
    }

 public function sendInvoice(Invoice $invoice)
 {
     $invoice->load('invitations', 'client', 'account');
     $entityType = $invoice->getEntityType();
     $view = 'invoice';
     $subject = trans("texts.{$entityType}_subject", ['invoice' => $invoice->invoice_number, 'account' => $invoice->account->getDisplayName()]);
     $accountName = $invoice->account->getDisplayName();
     $emailTemplate = $invoice->account->getEmailTemplate($entityType);
     $invoiceAmount = Utils::formatMoney($invoice->getRequestedAmount(), $invoice->client->currency_id);
     foreach ($invoice->invitations as $invitation) {
         if (!$invitation->user || !$invitation->user->email) {
             return false;
         }
         if (!$invitation->contact || !$invitation->contact->email) {
             return false;
         }
         $invitation->sent_date = \Carbon::now()->toDateTimeString();
         $invitation->save();
         $variables = ['$footer' => $invoice->account->getEmailFooter(), '$link' => $invitation->getLink(), '$client' => $invoice->client->getDisplayName(), '$account' => $accountName, '$contact' => $invitation->contact->getDisplayName(), '$amount' => $invoiceAmount];
         $data['body'] = str_replace(array_keys($variables), array_values($variables), $emailTemplate);
         $data['link'] = $invitation->getLink();
         $data['entityType'] = $entityType;
         $data['invoice_id'] = $invoice->id;
         $fromEmail = $invitation->user->email;
         $this->sendTo($invitation->contact->email, $fromEmail, $accountName, $subject, $view, $data);
         Activity::emailInvoice($invitation);
     }
     if (!$invoice->isSent()) {
         $invoice->invoice_status_id = INVOICE_STATUS_SENT;
         $invoice->save();
     }
     Event::fire(new InvoiceSent($invoice));
 }