麻烦if（$ _ SERVER ['REQUEST_METHOD'] ==“POST”）

please help me.

I have form like this in index.php

<form id="statusForm" enctype="multipart/form-data" method="post">
    <textarea name="statusText" role="textbox" id="wallpost"></textarea>
    <input id="photo_input" type="file" name="photo_input" />
    <input type="hidden" name="to_id" value="1" > 
    <button type="button" name="submit" onClick="write_wall_post();">
</form>
<div id="content"></div>

then i have .js file to handle this form

function write_wall_post()
{

var formData = new FormData($("#statusForm")[0]);
  $.ajax({
        type: "POST",
        url: "act_post_status.php",
        data: formData, 
        success: function(data){
            $("#wallpost").val("");             
            $("#photo_input").val('');
            var newStatus=data;
            $(newStatus).hide().prependTo("#content").fadeIn(2000);
        },
        processData: false,  // tell jQuery not to process the data
        contentType: false,
        cache:false
   });
}

and i have act_post_status.php to process this file submit

<?php 
//some configuration
if($_SERVER['REQUEST_METHOD'] == "POST")
{   
    //some variable declaration and image validation
    //Original Image
    if(move_uploaded_file($uploadedfile, $path.$time.'.'.$ext))
    {
        $is_image=1;
    }
    else
        echo "failed";
    }
}


 //inserting data to database
   $status = trim(strip_tags(htmlspecialchars($_POST["statusText"])));
   mysql_query("insert into news (status,is_image) values ('$status','$is_image')");
   echo "<div class='post'>$status</div>";
?>

the scenario i want is: when user input data (status), then click submit button, the content automatically show the update (handled by jquery)

but the fact is:

(1) when I completed the form (both status and picture), it works normally.

(2) but when I completed just data form (filling status input only), it was submitted to database successfully, but the content don't update automatically. I should refresh them to get the update.

(3) when i just filling the image input, it works normally like case (1).

Please help why if($_SERVER['REQUEST_METHOD'] == "POST") failed to echo the input by ajax request when data input (status) is blank/empty.

thousands of thanks. :)