Login password comparison question

The database saving field is md5 (password), and the password for form reception and processing is md5 (password suffix). Are the logical values ​​returned by these two judgments true? Why did I try last night and the account password returned was incorrect, but the ajax return value was ok. If all were entered randomly, ajax would return null. Why does ajax return ok if the passwords don't match? If there is a match, why is the incorrect password returned? What's the problem?
1.IndexController.class.php:

<code><?php namespace Home\Controller;
use Think\Controller;
class IndexController extends Controller {
  public function index(){
    $this->display();
  }
  public function checkUserName(){
    if(!IS_AJAX){
      $data=array('errMsg' => '非法访问方式');
    }
    $userName=I('username','','htmlspecialchars');
    $userPass=I('userpass','','htmlspecialchars');
    $userInfo=D("Stuser")->getUserInfo($userName);
    if($userInfo['userpass']!=handleMd5Pass($userPass)){
      //echo $userInfo['userpass']."<br>";
      //echo handleMd5Pass($userPass);
      echo "用户名或密码不正确";
      //var_dump(handleMd5Pass($userPass));
    }
    if($userInfo){
      //$userInfo->where(array('id' => $userInfo['id']))->save($errMsg);
      session('userId',$userInfo['id']);
      $data=array(
        'info' => 'ok',
        'callback' => U('/stfjzd-13/index.php/Home/Index/index')
      );
    }
    $this->ajaxReturn($data);
  }

}
</code>

2.StuserModel.class.php

<code><?php namespace Home\Model;
  use Think\Model;
  //用户表模型
  class StuserModel extends Model{
    private $DB="";
    public function __construct(){
      //构造函数创建模型
      $this->DB=M("Stuser");
    }
    //数据库中检索用户数据，find()检索一条->Index
    public function getUserInfo($userName){
      //$res=$this->DB->field('username','userpass')->where('username="'.$userName.'"')->find();
      $userInfo=$this->DB->where('username="'.$userName.'"')->find();
      echo $this->DB->getLastSql();
      return $userInfo;
    }
  }
?>
</code>

3.Login.js

<code>$('.search_sub').click(function(event){
  event.preventDefault();
  var userName=$("#username").val();
  var userPass=$("#userpass").val();
  if(userPass=="" || userName==""){
    alert("登陆名称与密码不能为空");
    $("#username").focus();
    return false;
  }else{
    var url="/stfjzd-13/index.php/Home/Index/checkUserName";
    //var url="{U('/stfjzd-13/index.php/Home/Index/checkUserName')}";
    $.post(url,{username:userName,userpass:userPass},function(msg){
      if(msg.errMsg=="ok"){
        window.location.href=msg.callback;
      }else{
        alert(msg.errMsg);
      }
    },"JSON")
  }
})
</code>

Reply content:

The database saving field is md5 (password), and the password for form reception and processing is md5 (password suffix). Are the logical values ​​returned by these two judgments true? Why did I try last night and the account password returned was incorrect, but the ajax return value was ok. If all were entered randomly, ajax would return null. Why does ajax return ok if the passwords don't match? If there is a match, why is the incorrect password returned? What's the problem?
1.IndexController.class.php:

<code><?php namespace Home\Controller;
use Think\Controller;
class IndexController extends Controller {
  public function index(){
    $this->display();
  }
  public function checkUserName(){
    if(!IS_AJAX){
      $data=array('errMsg' => '非法访问方式');
    }
    $userName=I('username','','htmlspecialchars');
    $userPass=I('userpass','','htmlspecialchars');
    $userInfo=D("Stuser")->getUserInfo($userName);
    if($userInfo['userpass']!=handleMd5Pass($userPass)){
      //echo $userInfo['userpass']."<br>";
      //echo handleMd5Pass($userPass);
      echo "用户名或密码不正确";
      //var_dump(handleMd5Pass($userPass));
    }
    if($userInfo){
      //$userInfo->where(array('id' => $userInfo['id']))->save($errMsg);
      session('userId',$userInfo['id']);
      $data=array(
        'info' => 'ok',
        'callback' => U('/stfjzd-13/index.php/Home/Index/index')
      );
    }
    $this->ajaxReturn($data);
  }

}
</code>

2.StuserModel.class.php

<code><?php namespace Home\Model;
  use Think\Model;
  //用户表模型
  class StuserModel extends Model{
    private $DB="";
    public function __construct(){
      //构造函数创建模型
      $this->DB=M("Stuser");
    }
    //数据库中检索用户数据，find()检索一条->Index
    public function getUserInfo($userName){
      //$res=$this->DB->field('username','userpass')->where('username="'.$userName.'"')->find();
      $userInfo=$this->DB->where('username="'.$userName.'"')->find();
      echo $this->DB->getLastSql();
      return $userInfo;
    }
  }
?>
</code>

3.Login.js

<code>$('.search_sub').click(function(event){
  event.preventDefault();
  var userName=$("#username").val();
  var userPass=$("#userpass").val();
  if(userPass=="" || userName==""){
    alert("登陆名称与密码不能为空");
    $("#username").focus();
    return false;
  }else{
    var url="/stfjzd-13/index.php/Home/Index/checkUserName";
    //var url="{U('/stfjzd-13/index.php/Home/Index/checkUserName')}";
    $.post(url,{username:userName,userpass:userPass},function(msg){
      if(msg.errMsg=="ok"){
        window.location.href=msg.callback;
      }else{
        alert(msg.errMsg);
      }
    },"JSON")
  }
})
</code>

<code>    if($userInfo['userpass']!=handleMd5Pass($userPass)){
        //此处只echo了错误，但是没有返回到前端。
      echo "用户名或密码不正确";
      //var_dump(handleMd5Pass($userPass));
    }
    if($userInfo){
        //你的代码运行到这里，用户名对，所以查出了userInfo，所以返回了ok。正确应该在密码验证错误的时候就返回给前端，不再往后运行
      session('userId',$userInfo['id']);
      $data=array(
        'info' => 'ok',
        'callback' => U('/stfjzd-13/index.php/Home/Index/index')
      );
    }
    $this->ajaxReturn($data);</code>

In comments

echo "Username or password is incorrect";

return here