รบกวนผู้รู้ แปลภาษาอังกฤษแล้วช่วยดู code ให้ทีค่ะ

<?php
// Multiple select lists - http://coursesweb.net/ajax/
if(!isset($_SESSION)) session_start();

// Here add your own data for connecting to MySQL database
$server = 'localhost';
$user = 'root';
$pass = '';
$dbase = 'sarabun';

// Here add the name of the table and columns that will be used for select lists, in their order
// Add null for 'links' if you don`t want to display their data too
$table = 'sites';
$ar_cols = array('dep', 'unit', 'categ', 'links');


$preid = 'slo_';        // a prefix used for element's ID, in which Ajax will add <select>
$col = $ar_cols[0];     // the variable used for the column that wil be selected
$re_html = '';          // will store the returned html code

// if there is data sent via POST, with index 'col' and 'wval'
if(isset($_POST['col']) && isset($_POST['wval'])) {
  // set the $col that will be selected and the value for WHERE (delete tags and external spaces in $_POST)
  $col = trim(strip_tags($_POST['col']));
  $wval = "'".trim(strip_tags($_POST['wval']))."'";
}

$key = array_search($col, $ar_cols);            // get the key associated with the value of $col in $ar_cols
$wcol = $key===0 ? $col : $ar_cols[$key-1];     // gets the column for the WHERE clause
$_SESSION['ar_cols'][$wcol] = isset($wval) ? $wval : $wcol;    // store in SESSION the column and its value for WHERE
  
// gets the next element in $ar_cols (needed in the onchange() function in <select> tag)
$last_key = count($ar_cols)-1;
$next_col = $key<$last_key ? $ar_cols[$key+1] : '';

$conn = new mysqli($server, $user, $pass, $dbase);     // connect to the MySQL database
mysqli_set_charset($conn, "utf8");

if (mysqli_connect_errno()) { exit('Connect failed: '. mysqli_connect_error()); }     // check connection

// sets an array with data of the WHERE condition (column=value) for SELECT query
for($i=1; $i<=$key; $i++) {
  $ar_where[] = '`'.$ar_cols[$i-1].'`='.$_SESSION['ar_cols'][$ar_cols[$i-1]];
}

// define a string with the WHERE condition, and then the SELECT query
$where = isset($ar_where) ? ' WHERE '. implode($ar_where, ' AND ') : '';
$sql = "SELECT DISTINCT `$col` FROM `$table`".$where;

$result = $conn->query($sql);       // perform the query and store the result

// if the $result contains at least one row
if ($result->num_rows > 0) {
  // sets the "onchange" event, which is added in <select> tag
  $onchg = $next_col!==null ? " onchange=\"ajaxReq('$next_col', this.value);\"" : '';

  // sets the select tag list (and the first <option>), if it's not the last column
  if($col!=$ar_cols[$last_key]) $re_html = $col. ': <select name="'. $col. '"'. $onchg. '><option>- - -</option>';

  while($row = $result->fetch_assoc()) {
    // if its the last column, reurns its data, else, adds data in OPTION tags
    if($col==$ar_cols[$last_key]) $re_html .= '<br/>'. $row[$col];
    else $re_html .= '<option value="'. $row[$col]. '">'. $row[$col]. '</option>';
  }

  if($col!=$ar_cols[$last_key]) $re_html .= '</select> ';        // ends the Select list
}
else { $re_html = '0 results'; }

$conn->close();

// if the selected column, $col, is the first column in $ar_cols
if($col==$ar_cols[0]) {
  // adds html code with SPAN (or DIV for last item) where Ajax will add the select dropdown lists
  // with ID in each SPAN, according to the columns added in $ar_cols
  for($i=1; $i<count($ar_cols); $i++) {
    if($ar_cols[$i]===null) continue;
    if($i==$last_key) $re_html .= '<div id="'. $preid.$ar_cols[$i]. '"> </div>';
    else $re_html .= '<span id="'. $preid.$ar_cols[$i]. '"> </span>';
  }

  // adds the columns in JS (used in removeLists() to remove the next displayed lists when makes other selects)
  $re_html .= '<script type="text/javascript">var ar_cols = '.json_encode($ar_cols).'; var preid = "'. $preid. '";</script>';
}
else echo $re_html;

// Multiple select lists - http://coursesweb.net/ajax/

// function used to remove the next lists already displayed when it chooses other options
function removeLists(colid) {
  var z = 0;
  // removes data in elements with the id stored in the "ar_cols" variable
  // starting with the element with the id value passed in colid
  for(var i=1; i<ar_cols.length; i++) {
    if(ar_cols[i]==null) continue;
    if(ar_cols[i]==colid) z = 1;
    if(z==1) document.getElementById(preid+ar_cols[i]).innerHTML = '';
  }
}

// sends data to a php file, via POST, and displays the received answer
function ajaxReq(col, wval) {
  removeLists(col);           // removes the already next selects displayed

  // if the value of wval is not '- - -' and '' (the first option)
  if(wval!='- - -' && wval!='') {
    var request = (window.XMLHttpRequest) ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');  // XMLHttpRequest instance
    var php_file = 'select_list.php';     // path and name of the php file

    // create pairs index=value with data that must be sent to server
    var  data_send = 'col='+col+'&wval='+wval;

    request.open("POST", php_file, true);			// set the request

    document.getElementById(preid+col).innerHTML = 'Loadding...';   // display a loading notification

    // adds a header to tell the PHP script to recognize the data as is sent via POST
    request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    request.send(data_send);	      	// calls the send() method with data_send

    // Check request status
    // If the response is received completely, will be added into the tag with id value of "col"
    request.onreadystatechange = function() {
      if (request.readyState==4) {
        document.getElementById(preid+col).innerHTML = request.responseText;
      }
    }
  }
}