Convert from ISO-8859-1 to UTF-8 and vice-versa; test if it's valid UTF-8

latin1-utf8.php

<?php

// ISO-8859-1 → UTF-8
function latin1ToUtf8(string $str): string
{
    return iconv('ISO-8859-1', 'UTF-8', $str);
}

// UTF-8 → ISO-8859-1
function utf8ToLatin1(string $str): string
{
    return iconv('UTF-8', 'ISO-8859-1', $str);
}

function isValidUtf8(string $str): bool
{
    return mb_check_encoding($str, 'UTF-8');
}