This function is strictly equivalent to rawurlencode().
Internally it uses curl_easy_escape() from libcurl, whose doc says: "This function converts the given input string to an URL encoded string (…). All input characters that are not a-z, A-Z, 0-9, '-', '.', '_' or '~' are converted to their "URL escaped" version (%NN where NN is a two-digit hexadecimal number)."