This code maps ISO 3166-1 alpha-2 country codes to their ISO 3166-1 numeric three-digit equivalents https://keithgreer.dev/global-payments-sca-3d-secure-2

numeric-country-code.php

/*
 * Usage: $countries['GB'] will return "826", the UK's ISO 3166-1 numeric three-digit code. 
 */

 $countries = [
    'AF' => '004',
    'AX' => '248',
    'AL' => '008',
    'DZ' => '012',
    'AS' => '016',
    'AD' => '020',
    'AO' => '024',
    'AI' => '660',
    'AQ' => '010',
    'AG' => '028',
    'AR' => '032',
    'AM' => '051',
    'AW' => '533',
    'AU' => '036',
    'AT' => '040',
    'AZ' => '031',
    'BS' => '044',
    'BH' => '048',
    'BD' => '050',
    'BB' => '052',
    'BY' => '112',
    'BE' => '056',
    'BZ' => '084',
    'BJ' => '204',
    'BM' => '060',
    'BT' => '064',
    'BO' => '068',
    'BQ' => '535',
    'BA' => '070',
    'BW' => '072',
    'BV' => '074',
    'BR' => '076',
    'IO' => '086',
    'BN' => '096',
    'BG' => '100',
    'BF' => '854',
    'BI' => '108',
    'KH' => '116',
    'CM' => '120',
    'CA' => '124',
    'CV' => '132',
    'KY' => '136',
    'CF' => '140',
    'TD' => '148',
    'CL' => '152',
    'CN' => '156',
    'CX' => '162',
    'CC' => '166',
    'CO' => '170',
    'KM' => '174',
    'CG' => '178',
    'CD' => '180',
    'CK' => '184',
    'CR' => '188',
    'CI' => '384',
    'HR' => '191',
    'CU' => '192',
    'CW' => '531',
    'CY' => '196',
    'CZ' => '203',
    'DK' => '208',
    'DJ' => '262',
    'DM' => '212',
    'DO' => '214',
    'EC' => '218',
    'EG' => '818',
    'SV' => '222',
    'GQ' => '226',
    'ER' => '232',
    'EE' => '233',
    'ET' => '231',
    'FK' => '238',
    'FO' => '234',
    'FJ' => '242',
    'FI' => '246',
    'FR' => '250',
    'GF' => '254',
    'PF' => '258',
    'TF' => '260',
    'GA' => '266',
    'GM' => '270',
    'GE' => '268',
    'DE' => '276',
    'GH' => '288',
    'GI' => '292',
    'GR' => '300',
    'GL' => '304',
    'GD' => '308',
    'GP' => '312',
    'GU' => '316',
    'GT' => '320',
    'GG' => '831',
    'GN' => '324',
    'GW' => '624',
    'GY' => '328',
    'HT' => '332',
    'HM' => '334',
    'VA' => '336',
    'HN' => '340',
    'HK' => '344',
    'HU' => '348',
    'IS' => '352',
    'IN' => '356',
    'ID' => '360',
    'IR' => '364',
    'IQ' => '368',
    'IE' => '372',
    'IM' => '833',
    'IL' => '376',
    'IT' => '380',
    'JM' => '388',
    'JP' => '392',
    'JE' => '832',
    'JO' => '400',
    'KZ' => '398',
    'KE' => '404',
    'KI' => '296',
    'KP' => '408',
    'KR' => '410',
    'KW' => '414',
    'KG' => '417',
    'LA' => '418',
    'LV' => '428',
    'LB' => '422',
    'LS' => '426',
    'LR' => '430',
    'LY' => '434',
    'LI' => '438',
    'LT' => '440',
    'LU' => '442',
    'MO' => '446',
    'MK' => '807',
    'MG' => '450',
    'MW' => '454',
    'MY' => '458',
    'MV' => '462',
    'ML' => '466',
    'MT' => '470',
    'MH' => '584',
    'MQ' => '474',
    'MR' => '478',
    'MU' => '480',
    'YT' => '175',
    'MX' => '484',
    'FM' => '583',
    'MD' => '498',
    'MC' => '492',
    'MN' => '496',
    'ME' => '499',
    'MS' => '500',
    'MA' => '504',
    'MZ' => '508',
    'MM' => '104',
    'NA' => '516',
    'NR' => '520',
    'NP' => '524',
    'NL' => '528',
    'NC' => '540',
    'NZ' => '554',
    'NI' => '558',
    'NE' => '562',
    'NG' => '566',
    'NU' => '570',
    'NF' => '574',
    'MP' => '580',
    'NO' => '578',
    'OM' => '512',
    'PK' => '586',
    'PW' => '585',
    'PS' => '275',
    'PA' => '591',
    'PG' => '598',
    'PY' => '600',
    'PE' => '604',
    'PH' => '608',
    'PN' => '612',
    'PL' => '616',
    'PT' => '620',
    'PR' => '630',
    'QA' => '634',
    'RE' => '638',
    'RO' => '642',
    'RU' => '643',
    'RW' => '646',
    'BL' => '652',
    'SH' => '654',
    'KN' => '659',
    'LC' => '662',
    'MF' => '663',
    'PM' => '666',
    'VC' => '670',
    'WS' => '882',
    'SM' => '674',
    'ST' => '678',
    'SA' => '682',
    'SN' => '686',
    'RS' => '688',
    'SC' => '690',
    'SL' => '694',
    'SG' => '702',
    'SX' => '534',
    'SK' => '703',
    'SI' => '705',
    'SB' => '090',
    'SO' => '706',
    'ZA' => '710',
    'GS' => '239',
    'SS' => '728',
    'ES' => '724',
    'LK' => '144',
    'SD' => '729',
    'SR' => '740',
    'SJ' => '744',
    'SZ' => '748',
    'SE' => '752',
    'CH' => '756',
    'SY' => '760',
    'TW' => '158',
    'TJ' => '762',
    'TZ' => '834',
    'TH' => '764',
    'TL' => '626',
    'TG' => '768',
    'TK' => '772',
    'TO' => '776',
    'TT' => '780',
    'TN' => '788',
    'TR' => '792',
    'TM' => '795',
    'TC' => '796',
    'TV' => '798',
    'UG' => '800',
    'UA' => '804',
    'AE' => '784',
    'GB' => '826',
    'US' => '840',
    'UM' => '581',
    'UY' => '858',
    'UZ' => '860',
    'VU' => '548',
    'VE' => '862',
    'VN' => '704',
    'VG' => '092',
    'VI' => '850',
    'WF' => '876',
    'EH' => '732',
    'YE' => '887',
    'ZM' => '894',
    'ZW' => '716',
 ];