Your static html files should be something like this:

<html>
    <body>
        <div class='header'>
        This is my header
        </div>
        <div class='main'>
            
            Contents written here can be edited
             
        </div>
    </body>
</html>

Note the special tags in bold. They are used to separate the editable from the non-editable text.

Copy the source code below and save it in a php file, say admin.php.

-------------------------------------------

<?php

if (isset($_REQUEST['logout'])) {
session_unset();
}

if (isset($_POST['submitUpdate'])) {
    if (get_magic_quotes_gpc()) {
        $_POST = array_map('stripslashes',$_POST);
   }
   $fc = file_get_contents($_POST['file']);
   // truncate file
   $fw = fopen($_POST['file'], 'w+');
   $text = explode("",$fc);
   $newText = $text[0]."".htmlentities($_POST['content'])."<!--EDITABLE ->".$text[2];
if (fwrite($fw, $newText)===FALSE) {
      die("Cannot write to file.");
   }
fclose($fw);
   exit("<div><span class='redText'>The file has been updated. Click <a href=\"index.php\">here</a> to go back to admin page.</div>");
}

if (isset($_POST['Submit'])) {
    if (($_POST['username'] == 'admin') && ($_POST['passwd'] == 'yourpassword')) {
      $_SESSION['username'] = 'login';
    }
   else {
     echo "<b>You login details is not correct. Pls login again</b>";
}
}

if ($_SESSION['username']=='login') {
   if (isset($_REQUEST['file'])) {
       $fc = file_get_contents($_REQUEST['file']);
       $text = explode("",$fc);
       echo "<form method='post' action=''><textarea name='content'>$text[1]</textarea>";
       echo "<p><input type='hidden' name='file' value='".$_REQUEST['file']."' /><input name='submitUpdate' type='submit' value='Update Page'></form>";
   }
   else {
      // edit to link to your own static html files
      echo "<p align='center'>
       <a href="?file=../index.html">Home Page</a><br/>
       <a href="?file=../contact_us.html">Contact Us</a><br/>
       <br/>
       <em>Click on the links above to edit the files.</em><br/>
      <a href="?logout">logout</a></p>";
   }
}
?>

<form method="post" action="">
<table width="400" border="0" align="center" cellpadding="2" cellspacing="2">
       <tr>
      <td>Username: </td>
      <td><input type="text" name="username"></td>
    </tr>
    <tr>
      <td>Passwd: </td>
      <td><input type="password" name="passwd"></td>
    </tr>
    <tr>
      <td> </td>
      <td><input type="submit" name="Submit" value="Submit">   <input type="reset" name="reset" value="Reset">
</td>
    </tr>
</table>
</form>