Standard URL encode function?

Look at indy IdURI unit, it has two static methods in the TIdURI class for Encode/Decode the URL.

uses
  IdURI;

..
begin
  S := TIdURI.URLEncode(str);
//
  S := TIdURI.URLDecode(str);
end;

Another simple way of doing this is to use the HTTPEncode function in the HTTPApp unit - very roughly

Uses 
  HTTPApp;

function URLEncode(const s : string) : string;
begin
  result := HTTPEncode(s);
end

HTTPEncode is deprecated in Delphi 10.3 - 'Use TNetEncoding.URL.Decode'

Uses
  NetEncoding;

function URLEncode(const s : string) : string;
begin
  result := TNetEncoding.URL.Encode(s);
end

Another option, is to use the Synapse library which has a simple URL encoding method (as well as many others) in the SynaCode unit.

uses
  SynaCode;
..
begin
  s := EncodeUrl( str );
//
  s := DecodeUrl( str );
end;

In a non-dotnet environment, the Wininet unit provides access to Windows' WinHTTP encode function: InternetCanonicalizeUrl

I was also facing the same issue (Delphi 4).

I resolved the issue using below mentioned function:

function fnstUrlEncodeUTF8(stInput : widestring) : string;
  const
    hex : array[0..255] of string = (
     '%00', '%01', '%02', '%03', '%04', '%05', '%06', '%07',
     '%08', '%09', '%0a', '%0b', '%0c', '%0d', '%0e', '%0f',
     '%10', '%11', '%12', '%13', '%14', '%15', '%16', '%17',
     '%18', '%19', '%1a', '%1b', '%1c', '%1d', '%1e', '%1f',
     '%20', '%21', '%22', '%23', '%24', '%25', '%26', '%27',
     '%28', '%29', '%2a', '%2b', '%2c', '%2d', '%2e', '%2f',
     '%30', '%31', '%32', '%33', '%34', '%35', '%36', '%37',
     '%38', '%39', '%3a', '%3b', '%3c', '%3d', '%3e', '%3f',
     '%40', '%41', '%42', '%43', '%44', '%45', '%46', '%47',
     '%48', '%49', '%4a', '%4b', '%4c', '%4d', '%4e', '%4f',
     '%50', '%51', '%52', '%53', '%54', '%55', '%56', '%57',
     '%58', '%59', '%5a', '%5b', '%5c', '%5d', '%5e', '%5f',
     '%60', '%61', '%62', '%63', '%64', '%65', '%66', '%67',
     '%68', '%69', '%6a', '%6b', '%6c', '%6d', '%6e', '%6f',
     '%70', '%71', '%72', '%73', '%74', '%75', '%76', '%77',
     '%78', '%79', '%7a', '%7b', '%7c', '%7d', '%7e', '%7f',
     '%80', '%81', '%82', '%83', '%84', '%85', '%86', '%87',
     '%88', '%89', '%8a', '%8b', '%8c', '%8d', '%8e', '%8f',
     '%90', '%91', '%92', '%93', '%94', '%95', '%96', '%97',
     '%98', '%99', '%9a', '%9b', '%9c', '%9d', '%9e', '%9f',
     '%a0', '%a1', '%a2', '%a3', '%a4', '%a5', '%a6', '%a7',
     '%a8', '%a9', '%aa', '%ab', '%ac', '%ad', '%ae', '%af',
     '%b0', '%b1', '%b2', '%b3', '%b4', '%b5', '%b6', '%b7',
     '%b8', '%b9', '%ba', '%bb', '%bc', '%bd', '%be', '%bf',
     '%c0', '%c1', '%c2', '%c3', '%c4', '%c5', '%c6', '%c7',
     '%c8', '%c9', '%ca', '%cb', '%cc', '%cd', '%ce', '%cf',
     '%d0', '%d1', '%d2', '%d3', '%d4', '%d5', '%d6', '%d7',
     '%d8', '%d9', '%da', '%db', '%dc', '%dd', '%de', '%df',
     '%e0', '%e1', '%e2', '%e3', '%e4', '%e5', '%e6', '%e7',
     '%e8', '%e9', '%ea', '%eb', '%ec', '%ed', '%ee', '%ef',
     '%f0', '%f1', '%f2', '%f3', '%f4', '%f5', '%f6', '%f7',
     '%f8', '%f9', '%fa', '%fb', '%fc', '%fd', '%fe', '%ff');
 var
   iLen,iIndex : integer;
   stEncoded : string;
   ch : widechar;
 begin
   iLen := Length(stInput);
   stEncoded := '';
   for iIndex := 1 to iLen do
   begin
     ch := stInput[iIndex];
     if (ch >= 'A') and (ch <= 'Z') then
       stEncoded := stEncoded + ch
     else if (ch >= 'a') and (ch <= 'z') then
       stEncoded := stEncoded + ch
     else if (ch >= '0') and (ch <= '9') then
       stEncoded := stEncoded + ch
     else if (ch = ' ') then
       stEncoded := stEncoded + '+'
     else if ((ch = '-') or (ch = '_') or (ch = '.') or (ch = '!') or (ch = '*')
       or (ch = '~') or (ch = '\')  or (ch = '(') or (ch = ')')) then
       stEncoded := stEncoded + ch
     else if (Ord(ch) <= $07F) then
       stEncoded := stEncoded + hex[Ord(ch)]
     else if (Ord(ch) <= $7FF) then
     begin
        stEncoded := stEncoded + hex[$c0 or (Ord(ch) shr 6)];
        stEncoded := stEncoded + hex[$80 or (Ord(ch) and $3F)];
     end
     else
     begin
        stEncoded := stEncoded + hex[$e0 or (Ord(ch) shr 12)];
        stEncoded := stEncoded + hex[$80 or ((Ord(ch) shr 6) and ($3F))];
        stEncoded := stEncoded + hex[$80 or ((Ord(ch)) and ($3F))];
     end;
   end;
   result := (stEncoded);
 end;

source : Java source code

AFAIK you need to make your own.

Here is an example.

• HTTPEncode

I'd like to point out that if you care much more about correctness than about efficiency, the simplest you can do is hex encode every character, even if it's not strictly necessary.

Just today I needed to encode a few parameters for a basic HTML login form submission. After going through all the options, each with their own caveats, I decided to write this naive version that works perfectly:

function URLEncode(const AStr: string): string;
var
  LBytes: TBytes;
  LIndex: Integer;
begin
  Result := '';
  LBytes := TEncoding.UTF8.GetBytes(AStr);
  for LIndex := Low(LBytes) to High(LBytes) do
    Result := Result + '%' + IntToHex(LBytes[LIndex], 2);
end;