Ajax php file control

I ask the question yesterday: Ajax Php get file from server, how to detect the file is open

There is this case. I have many files(data) in the server. I use Ajax and php for Client browser to get the data from the server.

When a person opens a website and accesses a file from the server, at the same time, if another person(user) open this website, I want to find a way to detect that file is open, if so, I can load a different file for her.

I think this is not duplicate from PHP check if file is in use

Finally, I find a way to solve this:

when a person open the website, he assess one file from php(ajax), and in another directory he create a tmp file, when he finished (after clicking a button), I delete those tmp file. So Another person coming, he can check if the same name tmp file exists, if so, he could not open it. and get another file.(create a tmp file).

According to this idea, I first to get the fileName list from the server directory, and then random generate a file name, and then use ajax to check whether this file is exist or not.Problem is I can not get the status from the php why? Any suggestion is appreciated, thanks!

If not exist, flag = false, so I can load this file(when loading a file, I create a tmp file with the same name).

ajax for get file name list, generate a file and check the file exist status:

$.ajax({
    type: "GET",
    url: "getXmlFileName.php",  //getXmlFileName.php
    cache: false,
    success: function(result){
       if(result instanceof Array)
       {
           if(result.length!==0)
           {
               var randomnumber;
               var flag = true;
               while(flag) {
                   randomnumber = Math.floor(Math.random() * result.length);
                   str = result[randomnumber];
                   $.ajax({
                       url: 'checkFileStatus.php',
                       type: "POST",
                       data: {
                           name: str
                       },
                       dataType: "json",
                       success: function (data) {  
                           console.log(data);
                           flag = data;//Here I can not get the status, Why?
                           //flag = true;  But If I change to this
                       },
                       error: function (xhr, textStatus, errorThrown) {
                           console.log('ajax saving  file error...');
                           //flag = false;  But If I change to this, I can get the value, Why?
                           return false;
                       }

                   });
              }

                   loader = new XMLLoader(str);
                   loader.load(str);
           }
       }
    }
});

getXmlFileName.php

  <?php

$files = array();
$dir = getcwd();
$dir = dirname($dir);  //get Parent directory
$dir .= "/XMLdata";  //jsondata
// Open a directory, and read its contents
if (is_dir($dir)){
  if ($dh = opendir($dir)){
    while (($file = readdir($dh)) !== false){
      if ($file == '.' || $file == '..') {
        continue;
    }
    $files[] = $file;

    }
    closedir($dh);
  }
}
    header('Content-type: application/json');
    echo json_encode($files);
?>

'checkFileStatus.php'

   <?php
    header("Content-type: application/json");
   $file_x ="./tmpData/";
   $file_x .= $_POST['name'];
   if (file_exists($file_x))
    {
        echo true;
    }
    else
    {
        echo false;
    }
?>

XMLLoader.load function:

    load: function (url) {
    var _this = this;
    $.ajax({
        type: "GET",
        url: url,
        dataType: "text",

        success: function (xml) {
            if (typeof xml == 'string' || xml instanceof String) {
                xml = xml.replace(/\\'/g, "'"); //dataType: "text",
                xml = xml.replace(/\\"/g, '"'); //dataType: "text",
                _this.parse($(xml));
            }
        }
    });
    this.saveTmpData();
},

saveTmpData

saveTmpData: function(){
    var _this = this;
    $.ajax({
        url: 'createTmpFile.php',
        type: "POST",  // type should be POST
        data: {
            name: _this.fileName
        }, // send the string directly
        dataType: "json",
        success: function (data) {
            console.log(data);
            return true;
        },
        complete: function () {
        },
        error: function (xhr, textStatus, errorThrown) {
            console.log('ajax saving json file error...');
            return false;
        }
    });
},

createTmpFile.php

   <?php
   header("Content-type: application/json");
   $file_x ="./tmpData/";
   $file_x .= $_POST['name'];
   $createFile = touch($file_x);
   echo (file_exists($file_x));
   ?>

Problem Solved

It seems I can continue, If I change code below, although I do not know why? Can anyone interpret this?

$.ajax({
                       url: 'checkFileStatus.php',
                       type: "POST",
                       data: {
                           name: str
                       },
                       dataType: "json",
                       success: function (data) {  
                           flag = true;  But If I change to this
                       },
                       error: function (xhr, textStatus, errorThrown) {
                           flag = false;
                       }

                   });