add an array to an array using arrayName[]= add_array without the index 0 in php [duplicate]

how can I add this array

    $triggers = [

    SMS_INVOICE => [
        'id' => 1,
        'name'        => 'Invoice Overdue Notice',
        'info'         => 'Trigger when invoice overdue notice is sent to customer contacts.',
    ]

this function without editing the function

   public function add_trigger($trigger)
{
   $this->triggers[] = $trigger;
}

the result will be

    array (size=10)
  'invoice_overdue_notice' => 
    array (size=3)
      'merge_fields' => 
        array (size=12)
          'id' => string '1' (length=1)
      'label' => string 'Invoice Overdue Notice' (length=22)
      'info' => string 'Trigger when invoice overdue notice is sent to customer contacts.' (length=65)
  'invoice_payment_recorded' => 

without [0] in front of it