php ajax file upload with chunk

I have php ajax file upload code but i need turn this code to chunk upload due to file size

upload.php
if(count($_FILES["file"]["name"]) > 0)
{
//$output = '';
sleep(3);
for($count=0; $count<count($_FILES["file"]["name"]); $count++)
{
$un_id = $_POST['un_id'];

$file_name = $_FILES["file"]["name"][$count];
$tmp_name = $_FILES["file"]['tmp_name'][$count];
$file_array = explode(".", $file_name);
$file_extension = end($file_array);
if(file_already_uploaded($file_name, $pdo))
{
$file_name = $file_array[0] . '-duplicated'.'-'.rand(1, 1000) . '.' . $file_extension;
}
$location = 'uploads/' . $file_name;
if(move_uploaded_file($tmp_name, $location))
{
$query = "
INSERT INTO tbl_image (image_name, image_description,un_id)
VALUES ('".$file_name."', '','".$un_id."')
";
$statement = $pdo->prepare($query);
$statement->execute();
}
}
}

function file_already_uploaded($file_name, $pdo)
{

$query = "SELECT * FROM tbl_image WHERE image_name = '".$file_name."'";
$statement = $pdo->prepare($query);
$statement->execute();
$number_of_rows = $statement->rowCount();
if($number_of_rows > 0)
{
return true;
}
else
{
return false;
}
}