1. Assess whether it's functionally correct or not, does it generate good cookies? I don't know if it's necessary for it to repeat the expires/domain/path/secure/httponly for each cookie.
2. Help reduce the bloat. I don't like how: (a) it calls itself 3 times. (b) it has the following code twice: $keyPrefix?"$keyPrefix\[$key\]":$key (c) in the middle, it checks: if (is_object($val) || is_array($val)) - it seems a bit redundant.

function http_build_cookie($data, $useJson=true, $expires = 0, $path = '', $domain = '', $secure = false, $httpOnly = false, $keyPrefix = '') {
    if (is_object($data)) {
        $data = (array) $data;
    }
    if (is_array($data)) {
        $cookie_parts = array();
        foreach ($data as $key => $val) {
            if ($useJson)
            {
                $cookie_parts[] = http_build_cookie(
                    rawurlencode($key)."="
                    .rawurlencode(json_encode($val)),
                    $useJson, $expires, $path, $domain, $secure, $httpOnly
                );
            }
            else
            {
                if (is_object($val) || is_array($val)) {
                    $cookie_parts[] = http_build_cookie(
                        $val, $useJson, $expires, $path, $domain,
                        $secure, $httpOnly, $keyPrefix?"$keyPrefix\[$key\]":$key
                    );
                }
                else
                {
                    $cookie_parts[] = http_build_cookie(
                        rawurlencode($keyPrefix?"$keyPrefix\[$key\]":$key)."="
                        .rawurlencode($val),
                        $useJson, $expires, $path, $domain, $secure, $httpOnly
                    );
                }
            }
        }
        return implode('; ', $cookie_parts);
    }
    if (is_scalar($data)) {
        $cookie = $data;
        if ($expires) {
            $cookie .= '; expires=' . gmdate('D, d-M-Y H:i:s T', $expires);
        }
        if ($path) {
            $cookie .= '; path=' . $path;
        }
        if ($domain) {
            $cookie .= '; domain=' . $domain;
        }
        if ($secure) {
            $cookie .= '; secure';
        }
        if ($httpOnly) {
            $cookie .= '; HttpOnly';
        }
        return $cookie;
    }
    return '';
}

1. Assess whether it's functionally correct or not, does it generate good cookies? I don't know if it's necessary for it to repeat the expires/domain/path/secure/httponly for each cookie.
2. Help reduce the bloat. I don't like how: (a) it calls itself 3 times. (b) it has the following code twice: $keyPrefix?$keyPrefix."[".$key."]":$key (c) in the middle, it checks: if (is_object($val) || is_array($val)) - it seems a bit redundant.

function http_build_cookie($data, $useJson=true, $expires = 0, $path = '', $domain = '', $secure = false, $httpOnly = false, $keyPrefix = '') {
    if (is_object($data)) {
        $data = (array) $data;
    }
    if (is_array($data)) {
        $cookie_parts = array();
        foreach ($data as $key => $val) {
            if ($useJson)
            {
                $cookie_parts[] = http_build_cookie(
                    rawurlencode($key)."="
                    .rawurlencode(json_encode($val)),
                    $useJson, $expires, $path, $domain, $secure, $httpOnly
                );
            }
            else
            {
                if (is_object($val) || is_array($val)) {
                    $cookie_parts[] = http_build_cookie(
                        $val, $useJson, $expires, $path, $domain,
                        $secure, $httpOnly, $keyPrefix?$keyPrefix."[".$key."]":$key
                    );
                }
                else
                {
                    $cookie_parts[] = http_build_cookie(
                        rawurlencode($keyPrefix?$keyPrefix."[".$key."]":$key)."="
                        .rawurlencode($val),
                        $useJson, $expires, $path, $domain, $secure, $httpOnly
                    );
                }
            }
        }
        return implode('; ', $cookie_parts);
    }
    if (is_scalar($data)) {
        $cookie = $data;
        if ($expires) {
            $cookie .= '; expires=' . gmdate('D, d-M-Y H:i:s T', $expires);
        }
        if ($path) {
            $cookie .= '; path=' . $path;
        }
        if ($domain) {
            $cookie .= '; domain=' . $domain;
        }
        if ($secure) {
            $cookie .= '; secure';
        }
        if ($httpOnly) {
            $cookie .= '; HttpOnly';
        }
        return $cookie;
    }
    return '';
}