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is it well-written?

Well, not really... it is very redundant. A commonly accepted principle is the Don't Repeat Yourself. principle. For example, the following block:

    if (getenv('HTTP_CLIENT_IP'))
        $mainIp = getenv('HTTP_CLIENT_IP');
    else if(getenv('HTTP_X_FORWARDED_FOR'))
        $mainIp = getenv('HTTP_X_FORWARDED_FOR');
    else if(getenv('HTTP_X_FORWARDED'))
        $mainIp = getenv('HTTP_X_FORWARDED');
    else if(getenv('HTTP_FORWARDED_FOR'))
        $mainIp = getenv('HTTP_FORWARDED_FOR');
    else if(getenv('HTTP_FORWARDED'))
        $mainIp = getenv('HTTP_FORWARDED');
    else if(getenv('REMOTE_ADDR'))
        $mainIp = getenv('REMOTE_ADDR');
    else
        $mainIp = 'UNKNOWN';
    return $mainIp;

Could be simplified by putting each key into an array:

$ipKeys = ['HTTP_CLIENT_IP', 'HTTP_X_FORWARDED_FOR', ...];

or that could be stored in a constant:

const IP_KEYS = ['HTTP_CLIENT_IP', 'HTTP_X_FORWARDED_FOR', ...];

Then iterate over those keys to set the value once a value is found:

foreach ($ipKeys as $key) {
    if (getenv($key)) {
        return getenv($key);
    }
}
return 'UNKNOWN';

As Gwyn Evans suggests the return value of the call to getenv() can be stored in a variable and if that value doesn't evaluate to false then the variable can be returned, though this may go against an accepted coding standard/style guide so use with caution:

foreach ($ipKeys as $key) {
    if ($value = getenv($key)) {
        return $value;
    }
}
return 'UNKNOWN';