upload verander naam

<?php
$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$uploadOk = 1;
$imageFileType = pathinfo($target_file,PATHINFO_EXTENSION);
// Check if image file is a actual image or fake image
if(isset($_POST["submit"])) 
    {
        $check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);
        if($check !== false) 
            {
                //echo "File is an image - " . $check["mime"] . ".";
                $uploadOk = 1;
            }
        else 
            {
                echo "File is not an image.";
                $uploadOk = 0;
            }
    }
// Check file size
if ($_FILES["fileToUpload"]["size"] > 500000) 
    {
        echo "Sorry, your file is too large.";
        $uploadOk = 0;
    }
// Allow certain file formats
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg" && $imageFileType != "gif" ) 
    {
        echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
        $uploadOk = 0;
    }
// Check if $uploadOk is set to 0 by an error
if ($uploadOk == 0) 
    {
        echo "Sorry, your file was not uploaded.";
        // if everything is ok, try to upload file
    }
else 
    {
        if(file_exists($target_file))
            {$newfilename="$_POST[lastname]" . '&#95;' . "$_POST[firstname]".".".$extension;
   if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $new_filename)){
                        $name=basename( $_FILES["fileToUpload"]["name"]);
                        echo $name;
                    } 
                else { echo "Sorry, there was an error uploading your file.";
                    }	}
        else {
                if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) 
                    {
                        $name=basename( $_FILES["fileToUpload"]["name"]);
                        echo $name;} 
                else {
                        echo "Sorry, there was an error uploading your file.";
                    }    }    }
?>