php算法看不懂，请高手逐句给解释下吧！该怎么处理

 $MM_paramName = ""; 

// *** Go To Record and Move To Record: create strings for maintaining URL and Form parameters
// create the list of parameters which should not be maintained
$MM_removeList = "&index=";
if ($MM_paramName != "") $MM_removeList .= "&".strtolower($MM_paramName)."=";  
$MM_keepURL="";
$MM_keepForm="";
$MM_keepBoth="";
$MM_keepNone="";
// add the URL parameters to the MM_keepURL string
reset ($HTTP_GET_VARS);   //$HTTP_GET_VARS等价于$_GET。早起的php3/4版本都是用$HTTP_GET_VARS，不过到了后来版本$HTTP_GET_VARS被弃用了。改用$_GET 了。
while (list ($key, $val) = each ($HTTP_GET_VARS)) {   //遍历数组每项，键=$key,值=$val
$nextItem = "&".strtolower($key)."=";
if (!stristr($MM_removeList, $nextItem)) {   //如果MM_removeList中不存在$nextItem
$MM_keepURL .= "&".$key."=".urlencode($val);  //连接字符串
}
}
// add the URL parameters to the MM_keepURL string
if(isset($HTTP_POST_VARS)){  //$HTTP_POST_VARS同$HTTP_GET_VARS
reset ($HTTP_POST_VARS);  //重置$HTTP_POST_VARS,使其指针回到0
while (list ($key, $val) = each ($HTTP_POST_VARS)) {  //遍历，同上
$nextItem = "&".strtolower($key)."=";    //连接字符串
if (!stristr($MM_removeList, $nextItem)) {   //同上
$MM_keepForm .= "&".$key."=".urlencode($val);  //同上
}
}
}
// create the Form + URL string and remove the intial '&' from each of the strings
$MM_keepBoth = $MM_keepURL."&".$MM_keepForm;
if (strlen($MM_keepBoth) > 0) $MM_keepBoth = substr($MM_keepBoth, 1);   //如果$MM_keepBoth字节长度大于0，则从1字节位置截取到末尾
if (strlen($MM_keepURL) > 0) $MM_keepURL = substr($MM_keepURL, 1);    //类似上面
if (strlen($MM_keepForm) > 0) $MM_keepForm = substr($MM_keepForm, 1);   //类似上面