php to read only the numbers, avoid any letters or characters

I want to modify a part of the php file that generates my invoices: in this section: ------------------------------------------------------------------------------- include($path_to_root .'/control/[login to view URL]'); /*$numero_autorizacion = '29040011007'; $numero_factura = '1503'; $nit_cliente = '4189179011'; $monto_compra = '2500'; $clave = '9rCB7Sv4X29d)5k7N%3ab89p-3(5[A';*/ $total_cc = round($sub_total); $codigo_control = CodigoControlV7::generar($auth_no, $myrow["reference"], $myrow['tax_id'], $fecha_compra, $total_cc, $empresa['key_dos']); ----------------------------------------------------------------------------- I want the code to read only the numbers that exist in "reference". And avoid any letters or characters that may exist. As for example if the "reference" is: a0001 that the algorithm will only read 0001 and obviate the letter "a". This is not currently the case. As this page does (see the "Verhoeff Check Digit Calculator" part) [login to view URL] If you enter for example: a485 the result is: 9 If you enter: 485 the result is: 9