rename my image with a number then auto increment it

i have a simple problem. i just want to rename my image with a number then auto increment it. the output should be 1.jpg 2.jpg 3.jpg here is my code:

if(isset($_FILES['file_array'])){

    $name_array = $_FILES['file_array']['name'];
    $tmp_name_array = $_FILES['file_array']['tmp_name'];
    $type_array = $_FILES['file_array']['type'];
    $size_array = $_FILES['file_array']['size'];
    $error_array = $_FILES['file_array']['error'];

    $pid = mysql_insert_id();
    $foldername = "$pid";
    $dir = "../../img/inventory_images/$foldername/";
    mkdir($dir, 0777, true);

    for($i = 0; $i < count($tmp_name_array); $i++){
        if(move_uploaded_file($tmp_name_array[$i], "../../img/inventory_images/$foldername/".$name_array[$i])){

        }else{

        }
    }
}