This is a complete (AFAIK) array that includes all the countries recognized with ISO 3166-1 alpha-2 codes as used by Flagcdn.com. You can use these codes directly to reference flag images, for example, https://flagcdn.com/w320/us.png for the United States flag.
List converted from https://flagcdn.com/en/codes.json
Might need a little tweaking depending on its use. Sometimes country names don't match. It's usually counties with long names like Côte d'Ivoire (Ivory Coast).
function getCountryCode($countryName) {
$countryMap = [
'Andorra' => 'ad',
'United Arab Emirates' => 'ae',
'Afghanistan' => 'af',
'Antigua and Barbuda' => 'ag',
'Anguilla' => 'ai',
'Albania' => 'al',
'Armenia' => 'am',
'Angola' => 'ao',
'Antarctica' => 'aq',
'Argentina' => 'ar',
'American Samoa' => 'as',
'Austria' => 'at',
'Australia' => 'au',
'Aruba' => 'aw',
'Åland Islands' => 'ax',
'Azerbaijan' => 'az',
'Bosnia and Herzegovina' => 'ba',
'Barbados' => 'bb',
'Bangladesh' => 'bd',
'Belgium' => 'be',
'Burkina Faso' => 'bf',
'Bulgaria' => 'bg',
'Bahrain' => 'bh',
'Burundi' => 'bi',
'Benin' => 'bj',
'Saint Barthélemy' => 'bl',
'Bermuda' => 'bm',
'Brunei' => 'bn',
'Bolivia' => 'bo',
'Caribbean Netherlands' => 'bq',
'Brazil' => 'br',
'Bahamas' => 'bs',
'Bhutan' => 'bt',
'Bouvet Island' => 'bv',
'Botswana' => 'bw',
'Belarus' => 'by',
'Belize' => 'bz',
'Canada' => 'ca',
'Cocos (Keeling) Islands' => 'cc',
'DR Congo' => 'cd',
'Central African Republic' => 'cf',
'Republic of the Congo' => 'cg',
'Switzerland' => 'ch',
"Côte d'Ivoire (Ivory Coast)" => 'ci',
'Cook Islands' => 'ck',
'Chile' => 'cl',
'Cameroon' => 'cm',
'China' => 'cn',
'Colombia' => 'co',
'Costa Rica' => 'cr',
'Cuba' => 'cu',
'Cape Verde' => 'cv',
'Curaçao' => 'cw',
'Christmas Island' => 'cx',
'Cyprus' => 'cy',
'Czechia' => 'cz',
'Germany' => 'de',
'Djibouti' => 'dj',
'Denmark' => 'dk',
'Dominica' => 'dm',
'Dominican Republic' => 'do',
'Algeria' => 'dz',
'Ecuador' => 'ec',
'Estonia' => 'ee',
'Egypt' => 'eg',
'Western Sahara' => 'eh',
'Eritrea' => 'er',
'Spain' => 'es',
'Ethiopia' => 'et',
'European Union' => 'eu',
'Finland' => 'fi',
'Fiji' => 'fj',
'Falkland Islands' => 'fk',
'Micronesia' => 'fm',
'Faroe Islands' => 'fo',
'France' => 'fr',
'Gabon' => 'ga',
'United Kingdom' => 'gb',
'England' => 'gb-eng',
'Northern Ireland' => 'gb-nir',
'Scotland' => 'gb-sct',
'Wales' => 'gb-wls',
'Grenada' => 'gd',
'Georgia' => 'ge',
'French Guiana' => 'gf',
'Guernsey' => 'gg',
'Ghana' => 'gh',
'Gibraltar' => 'gi',
'Greenland' => 'gl',
'Gambia' => 'gm',
'Guinea' => 'gn',
'Guadeloupe' => 'gp',
'Equatorial Guinea' => 'gq',
'Greece' => 'gr',
'South Georgia' => 'gs',
'Guatemala' => 'gt',
'Guam' => 'gu',
'Guinea-Bissau' => 'gw',
'Guyana' => 'gy',
'Hong Kong' => 'hk',
'Heard Island and McDonald Islands' => 'hm',
'Honduras' => 'hn',
'Croatia' => 'hr',
'Haiti' => 'ht',
'Hungary' => 'hu',
'Indonesia' => 'id',
'Ireland' => 'ie',
'Israel' => 'il',
'Isle of Man' => 'im',
'India' => 'in',
'British Indian Ocean Territory' => 'io',
'Iraq' => 'iq',
'Iran' => 'ir',
'Iceland' => 'is',
'Italy' => 'it',
'Jersey' => 'je',
'Jamaica' => 'jm',
'Jordan' => 'jo',
'Japan' => 'jp',
'Kenya' => 'ke',
'Kyrgyzstan' => 'kg',
'Cambodia' => 'kh',
'Kiribati' => 'ki',
'Comoros' => 'km',
'Saint Kitts and Nevis' => 'kn',
'North Korea' => 'kp',
'South Korea' => 'kr',
'Kuwait' => 'kw',
'Cayman Islands' => 'ky',
'Kazakhstan' => 'kz',
'Laos' => 'la',
'Lebanon' => 'lb',
'Saint Lucia' => 'lc',
'Liechtenstein' => 'li',
'Sri Lanka' => 'lk',
'Liberia' => 'lr',
'Lesotho' => 'ls',
'Lithuania' => 'lt',
'Luxembourg' => 'lu',
'Latvia' => 'lv',
'Libya' => 'ly',
'Morocco' => 'ma',
'Monaco' => 'mc',
'Moldova' => 'md',
'Montenegro' => 'me',
'Saint Martin' => 'mf',
'Madagascar' => 'mg',
'Marshall Islands' => 'mh',
'North Macedonia' => 'mk',
'Mali' => 'ml',
'Myanmar' => 'mm',
'Mongolia' => 'mn',
'Macau' => 'mo',
'Northern Mariana Islands' => 'mp',
'Martinique' => 'mq',
'Mauritania' => 'mr',
'Montserrat' => 'ms',
'Malta' => 'mt',
'Mauritius' => 'mu',
'Maldives' => 'mv',
'Malawi' => 'mw',
'Mexico' => 'mx',
'Malaysia' => 'my',
'Mozambique' => 'mz',
'Namibia' => 'na',
'New Caledonia' => 'nc',
'Niger' => 'ne',
'Norfolk Island' => 'nf',
'Nigeria' => 'ng',
'Nicaragua' => 'ni',
'Netherlands' => 'nl',
'Norway' => 'no',
'Nepal' => 'np',
'Nauru' => 'nr',
'Niue' => 'nu',
'New Zealand' => 'nz',
'Oman' => 'om',
'Panama' => 'pa',
'Peru' => 'pe',
'French Polynesia' => 'pf',
'Papua New Guinea' => 'pg',
'Philippines' => 'ph',
'Pakistan' => 'pk',
'Poland' => 'pl',
'Saint Pierre and Miquelon' => 'pm',
'Pitcairn Islands' => 'pn',
'Puerto Rico' => 'pr',
'Palestine' => 'ps',
'Portugal' => 'pt',
'Palau' => 'pw',
'Paraguay' => 'py',
'Qatar' => 'qa',
'Réunion' => 're',
'Romania' => 'ro',
'Serbia' => 'rs',
'Russia' => 'ru',
'Rwanda' => 'rw',
'Saudi Arabia' => 'sa',
'Solomon Islands' => 'sb',
'Seychelles' => 'sc',
'Sudan' => 'sd',
'Sweden' => 'se',
'Singapore' => 'sg',
'Saint Helena, Ascension and Tristan da Cunha' => 'sh',
'Slovenia' => 'si',
'Svalbard and Jan Mayen' => 'sj',
'Slovakia' => 'sk',
'Sierra Leone' => 'sl',
'San Marino' => 'sm',
'Senegal' => 'sn',
'Somalia' => 'so',
'Suriname' => 'sr',
'South Sudan' => 'ss',
'São Tomé and Príncipe' => 'st',
'El Salvador' => 'sv',
'Sint Maarten' => 'sx',
'Syria' => 'sy',
'Eswatini (Swaziland)' => 'sz',
'Turks and Caicos Islands' => 'tc',
'Chad' => 'td',
'French Southern and Antarctic Lands' => 'tf',
'Togo' => 'tg',
'Thailand' => 'th',
'Tajikistan' => 'tj',
'Tokelau' => 'tk',
'Timor-Leste' => 'tl',
'Turkmenistan' => 'tm',
'Tunisia' => 'tn',
'Tonga' => 'to',
'Turkey' => 'tr',
'Trinidad and Tobago' => 'tt',
'Tuvalu' => 'tv',
'Taiwan' => 'tw',
'Tanzania' => 'tz',
'Ukraine' => 'ua',
'Uganda' => 'ug',
'United States Minor Outlying Islands' => 'um',
'United Nations' => 'un',
'United States' => 'us',
'Alaska' => 'us-ak',
'Alabama' => 'us-al',
'Arkansas' => 'us-ar',
'Arizona' => 'us-az',
'California' => 'us-ca',
'Colorado' => 'us-co',
'Connecticut' => 'us-ct',
'Delaware' => 'us-de',
'Florida' => 'us-fl',
'Georgia' => 'us-ga',
'Hawaii' => 'us-hi',
'Iowa' => 'us-ia',
'Idaho' => 'us-id',
'Illinois' => 'us-il',
'Indiana' => 'us-in',
'Kansas' => 'us-ks',
'Kentucky' => 'us-ky',
'Louisiana' => 'us-la',
'Massachusetts' => 'us-ma',
'Maryland' => 'us-md',
'Maine' => 'us-me',
'Michigan' => 'us-mi',
'Minnesota' => 'us-mn',
'Missouri' => 'us-mo',
'Mississippi' => 'us-ms',
'Montana' => 'us-mt',
'North Carolina' => 'us-nc',
'North Dakota' => 'us-nd',
'Nebraska' => 'us-ne',
'New Hampshire' => 'us-nh',
'New Jersey' => 'us-nj',
'New Mexico' => 'us-nm',
'Nevada' => 'us-nv',
'New York' => 'us-ny',
'Ohio' => 'us-oh',
'Oklahoma' => 'us-ok',
'Oregon' => 'us-or',
'Pennsylvania' => 'us-pa',
'Rhode Island' => 'us-ri',
'South Carolina' => 'us-sc',
'South Dakota' => 'us-sd',
'Tennessee' => 'us-tn',
'Texas' => 'us-tx',
'Utah' => 'us-ut',
'Virginia' => 'us-va',
'Vermont' => 'us-vt',
'Washington' => 'us-wa',
'Wisconsin' => 'us-wi',
'West Virginia' => 'us-wv',
'Wyoming' => 'us-wy',
'Uruguay' => 'uy',
'Uzbekistan' => 'uz',
'Vatican City (Holy See)' => 'va',
'Saint Vincent and the Grenadines' => 'vc',
'Venezuela' => 've',
'British Virgin Islands' => 'vg',
'United States Virgin Islands' => 'vi',
'Vietnam' => 'vn',
'Vanuatu' => 'vu',
'Wallis and Futuna' => 'wf',
'Samoa' => 'ws',
'Kosovo' => 'xk',
'Yemen' => 'ye',
'Mayotte' => 'yt',
'South Africa' => 'za',
'Zambia' => 'zm',
'Zimbabwe' => 'zw',
// Add more as needed
];
return $countryMap[$countryName] ?? 'unknown';
}