如何将上传的文件写入另一个文件

我需要什么:

当用户上传一个文件时，该文件的所有内容应该写入另一个文件。

我的代码

            $.validator.addMethod("custom_phone", function(value, element) {
            return this.optional(element) || value.match(/^[0-9,'+-]+$/);
            }, "Please enter a valid phone number.");
            $("#query").validate(
            {
            rules: { 
            mobile:
            {
            required: true
            },
            email:
            {
            required: true,
            email: true
            },
            name:
            {
            required: true
            }, 

            },
            errorPlacement: function (error, element) { 
            if (element.parent().hasClass("input-append")){
            error.insertAfter(element.parent());
            }else{
            error.insertAfter(element);
            } 
            },
            success: function(element) {
            element.closest('.control-group').removeClass('error').addClass('success');
            },
            submitHandler: function(form) {
            // $("#feedback").remove();
            //$('#feedback_ajax').removeClass('dis-non');
            var input =$("#query").serialize();
            alert(input);
            var myJsonString = JSON.stringify(input);
            alert(myJsonString);
            //console.log(myJsonString);
            $.ajax({
            url: "{{ DomainDetect() }}/ajax?for=formdata",
            type: "POST",
            crossDomain: true,
            cache: false,
            data: myJsonString,
            dataType: 'json',
            success: function(responseData, textStatus, jqXHR) {
            },
            error: function (responseData, textStatus, errorThrown) {
             return false;
            }
            }).done(function(msg ) {
                console.log(msg);
                data = $.parseJSON(responseData);
                console.log(data);
             $('.8u').empty();
             $('.row flush').append(msg);
            }); 
            }
            });

        <form id= "query" name ="query" class="regform" method="post" enctype="multipart/form-data">
    <div class="row orgrw flush pdfrm">
    <div class="3u"><label>Name</label></div>
    <div class="6u">
    <input class="text txtbx" type="text" name="name">
    </div>
    <div class="3u"><p class="alert alert_name">Required</p></div>
    </div>
    <div class="row orgrw flush pdfrm">
    <div class="3u"><label>Email</label></div>
    <div class="6u">
    <input class="text txtbx" type="text" name="email">
    </div>
    <div class="3u"><p class="alert alert_name">Required</p></div>
    </div>
    <div class="row orgrw flush pdfrm">
    <div class="3u"><label>Mobile No.</label></div> 
   <div class="6u">
   <input class="text txtbx" type="text" name="mobile">
   </div>
   <div class="3u"><p class="alert alert_name">Required</p></div>
</div>
<div class="row orgrw flush pdfrm">
<div class="3u"><label>Upload resume</label></div>
<div class="6u">
<input class="" type="file" name="file">
</div>
<div class="3u not-mobile">&nbsp;</div>
</div>
<div class="row orgrw flush pdfrm">
<div class="3u"><label style="line-height:1em">Any comments<br><span class="ft4">(optional)</span></label></div>
<div class="6u">
<textarea name="s_comment" rows="2" cols="10" style="min-height:6em; overflow:auto; padding:0.5em; font-size:16px; background-color:#f4f4f4; border-radius:0"></textarea>
</div>
<div class="3u not-mobile">&nbsp;</div>
</div>
<div class="row orgrw flush pdfrm">
<div class="3u not-mobile">&nbsp;</div>
<div class="6u aligncenter">
<input type="submit" class="attendnow cursr button" value="Submit">
</div>
<div class="3u not-mobile">&nbsp;</div>
</div>
</form>

            not working:echo $_FILES["file"];
           if(isset($_POST['file']))
            {
            $upload=$_POST['file'];
            $file="uploaddocument.txt";
            if(file_exists($file))
            {
            $current = file_get_contents($upload);
            }
             $sucess=file_put_contents($file, $current);
            }
            else
            {
              echo "fie not uploaded";
            }

Array(
    ["name"] => Deepak
    [email] => dpk.pandit1992@gmail.com
    [mobile] => 919871077792
    [s_comment] => "fesgdtfd"
)

Error.log

      Undefined index: file 

我面临的问题是我无法在另一个文件中写入上传文件数据的内容