public function getInvoiceNumberFormatted($id_lang, $id_shop = null)
{
//first %s is for prefix
//second %s is for Y (year) + m(month)
//then your text (you can include spaces if you need)
//%d is for invoice number
return sprintf('%s%sYOUR-FIXED-TEXT-HERE%6d', Configuration::get('PS_INVOICE_PREFIX', $id_lang, null, $id_shop), date('Ym', $this->date_add), $this->number);
}