Delphi2010 CallFunction:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    btn1: TButton;
    btn2: TButton;
    procedure btn1Click(Sender: TObject);
    procedure btn2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

function MyFunction(s:PAnsiChar):Integer;stdcall;external 'MyDll';

//function MyFunction(s:PChar):Integer;stdcall;external 'MyDll';

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.btn1Click(Sender: TObject);
begin
   MyFunction(PAnsiChar('hello')); //'hello'

  //MyFunction(PChar('hello')); //'h'

end;

end.

Delphi6 DllFunction:

library MyDll;

{ Important note about DLL memory management: ShareMem must be the
  first unit in your library's USES clause AND your project's (select
  Project-View Source) USES clause if your DLL exports any procedures or
  functions that pass strings as parameters or function results. This
  applies to all strings passed to and from your DLL--even those that
  are nested in records and classes. ShareMem is the interface unit to
  the BORLNDMM.DLL shared memory manager, which must be deployed along
  with your DLL. To avoid using BORLNDMM.DLL, pass string information
  using PChar or ShortString parameters. }

uses
  SysUtils,
  Classes,
  Dialogs;

{$R *.res}

function MyFunction(s:PChar):Integer;stdcall;export;
begin
  ShowMessage(s);
end;

exports MyFunction;
begin
end.