php ajax用户登录代码

function chk_login(form){
    var username=form.username;
    var password=form.password;
    if(username.value == ""){
        alert("用户名不能为空");
        username.focus();
        return false;
    }
    if(password.value == ""){
        alert("密码不能为空");
        password.focus();
        return false;
    }

    var url = 'login_chk.php?username='+username.value+'&password='+password.value;
    xmlhttp.open('get',url,true);
//为什么下面加了下面注释掉的代码后，会没反应了，没加就有反应，我的php文件路径都是正确的，也有引入xmlhttprequest.js
xmlhttp.onreadystatechange = function(){
        if(xmlhttp.readyState == 4){
            if(xmlhttp.status == 200){
                var msg = xmlhttp.responseText;
                if(msg == "1"){
                    alert("登陆成功");
                    window.location='index.php?name='+username.value;
                }else if((msg == "2"){
                    alert("登陆用户名或密码错误");
                }else{
                    alert(msg);
                }
            }
        }
    }
    xmlhttp.send(null);*/
}

ogin

<?php
//strstr:从左向右查找  返回值：字符串
//strrchr:从右向左查找 返回值：字符串
//strpos:从左向右查找  返回值：整型，假如查找的字符串不存在，则返回空
//strrpos:从右向左查找 返回值：整型
$str="天高任鸟飞，海阔凭鱼跃";//strstr:从左向右查找  strrchr:从右向左查找
echo "原始字符串：".$str."<br />";
echo "用strstr函数搜索“，”的返回结果：".strstr($str,"，")."<br>";
echo "用strstr函数搜索“鸟飞”的返回结果：".strstr($str,"鸟飞")."<br>";

$str1="I have a great dream.";
echo "原始字符串:".$str1."<br />";
echo "用strrchr函数搜索“e”的返回结果：".strrchr($str1,"e")."<br>";
echo "用strrchr函数搜索“ea”的返回结果：".strrchr($str1,"ve")."<br>";

$str2="I am an abstract about abroad.";
echo "原始字符串为：".$str2."<br>";
echo "ab在字符串中的第一次出现位置为：".strpos($str2,"ab")."<br>";
echo "ab在字符串中的第一次出现位置为：".strpos($str2,"am")."<br>";
echo "abcd在字符串中第一次出现的位置为：".strpos($str2,"aman")."<br>";

$str3="I am is wang hui.";
echo "原始字符串为：".$str3."<br>";
echo "I在字符串中的最后一次出现位置为：".strrpos($str3,"I")."<br>";
echo "an在字符串中的最后一次出现位置为：".strrpos($str3,"an")."<br>";
echo "i在字符串中最后一次出现的位置为：".strrpos($str3,"i");

?>

class mysql {
    private $db_host; //数据库主机
    private $db_user; //数据库用户名
    private $db_pwd; //数据库用户名密码
    private $db_database; //数据库名
    private $conn; //数据库连接标识;
    private $result; //执行query命令的结果资源标识
    private $sql; //sql执行语句
    private $row; //返回的条目数
    private $coding; //数据库编码，GBK,UTF8,gb2312
    private $bulletin = true; //是否开启错误记录
    private $show_error = true; //测试阶段，显示所有错误,具有安全隐患,默认关闭
    private $is_error = false; //发现错误是否立即终止,默认true,建议不启用，因为当有问题时用户什么也看不到是很苦恼的

    /*构造函数*/
    public function __construct($db_host, $db_user, $db_pwd, $db_database, $conn, $coding) {
        $this->db_host = $db_host;
        $this->db_user = $db_user;
        $this->db_pwd = $db_pwd;
        $this->db_database = $db_database;
        $this->conn = $conn;
        $this->coding = $coding;
        $this->connect();
    }

    /*数据库连接*/
    public function connect() {
        if ($this->conn == "pconn") {
            //永久链接
            $this->conn = mysql_pconnect($this->db_host, $this->db_user, $this->db_pwd);
        } else {
            //即使链接
            $this->conn = mysql_connect($this->db_host, $this->db_user, $this->db_pwd);
        }

        if (!mysql_select_db($this->db_database, $this->conn)) {
            if ($this->show_error) {
                $this->show_error("数据库不可用：", $this->db_database);
            }
        }
        mysql_query("SET NAMES $this->coding");
    }

    /*数据库执行语句，可执行查询添加修改删除等任何sql语句*/
    public function query($sql) {
        if ($sql == "") {
            $this->show_error("SQL语句错误：", "SQL查询语句为空");
        }
        $this->sql = $sql;

        $result = mysql_query($this->sql, $this->conn);

        if (!$result) {
            //调试中使用，sql语句出错时会自动打印出来
            if ($this->show_error) {
                $this->show_error("错误SQL语句：", $this->sql);
            }
        } else {
            $this->result = $result;
        }
        return $this->result;
    }

    /*创建添加新的数据库*/
    public function create_database($database_name) {
        $database = $database_name;
        $sqlDatabase = 'create database ' . $database;
        $this->query($sqlDatabase);
    }