php session_destroy()和session_unset()区别

<?php

function session_clean1($logout=false)
 {
  $v=array();
  foreach($_SESSION as $x=>$y)
   if($x!="redirector"&&($x!="user"||$logout))
    $v[]=$x;

  foreach($v as $x)
   unset($_SESSION[$x]);
  return;
 }

function session_clean2($logout=false)
 {
  foreach($_SESSION as $x=>$y)
   if($x!="redirector"&&($x!="user"||$logout))
    unset($_SESSION[$x]);
  return;
 }

function session_clean3($logout=false)
 {
  $s=($logout||!isset($_SESSION["user"]))?array():
   array("user"=>$_SESSION["user"]);
  if(isset($_SESSION["redirector"]))
   $s["redirector"]=$_SESSION["redirector"];
  $_SESSION=$s;
 }

?>

<?php
// Initialize the session.
// If you are using session_name("something"), don't forget it now!
session_start();

// Unset all of the session variables.
$_SESSION = array();

// If it's desired to kill the session, also delete the session cookie.
// Note: This will destroy the session, and not just the session data!
if (ini_get("session.use_cookies")) {
    $params = session_get_cookie_params();
    setcookie(session_name(), '', time() - 42000,
        $params["path"], $params["domain"],
        $params["secure"], $params["httponly"]
    );
}

// Finally, destroy the session.
session_destroy();
?>

<?php

$str="ni hao ,wo jiao wang hui ,lai zi an hui chao hu";

echo "原字符串:".$str."<br />";

$str1=substr_count($str,'i');//substr_count():统计字符串在函数中出现的次数

echo "使用substr_count()之后:".$str1."<br />";

$str2=substr_count($str,'i',6);//substr_count($str,'i',6):从第6个字符串开始统计，后面还跟数字的话，就是统计多少个，如果不加的话，默认是到最后

echo "使用substr_count()之后:".$str2."<br />";

?>

<?php
  include("file.php");
  $pagesize=5;
  $url=$_SERVER[REQUEST_URI];
  $url=parse_url($url);
  $url=$url[path];

  $a=mysql_query("select * from message");
  $b=mysql_num_rows($a);
   if($_GET[page]){
       $pagevel=$_GET[page];
       $page=($pagevel-1)*pagesize;
       $page.=',';
   }else{
       $page=1;   //这里加个未获取到值的处理
   }

   if($b>$pagesize){
       if($pagevel<=1) $pagevel=1;
       echo "共 $b 条 <a herf=$url?page="($pagevel-1)">上一页</a>" .
               " <a herf=$url?page="($pagevel+1)">下一页</a>";

   }
   $sql="select * from message limit $page ,$pagesize ";
   $query=mysql_query($sql);
   while($row=mysql_fetch_array($query)){
   echo "<hr><br>"."$row[title]"."$row[context]";

  }

?>

beforeSend : function (XMLHttpRequest) {

    XMLHttpRequest.setRequestHeader("X-Requested-With","XMLHttpRequest");

}；

if(!emptyempty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest')
{
//If AJAX Request Then
}else{
//something else
}

1，  如果