PHP - Trying To Add Time Stamp To File Name To Make It Unique

I'm having a problem with getting the timestamp to attach to the filename.  Currently when it's uploaded in the database it says $timestamp in front of the filename instead of the time.

Scroll down towards the bottom of the code to see it.  Thanks for your help!


$username = $_GET['username'];
$item_id = $_GET['id'];

  define( 'DESIRED_IMAGE_WIDTH', 150 );
  define( 'DESIRED_IMAGE_HEIGHT', 150 );

  $source_path = $_FILES[ 'thumb' ][ 'tmp_name' ];
  $timestamp = time();

  //
  // Add file validation code here
  //

  list( $source_width, $source_height, $source_type ) = getimagesize( $source_path );

  switch ( $source_type )
  {
    case IMAGETYPE_GIF:
      $source_gdim = imagecreatefromgif( $source_path );
      break;

    case IMAGETYPE_JPEG:
      $source_gdim = imagecreatefromjpeg( $source_path );
      break;

    case IMAGETYPE_PNG:
      $source_gdim = imagecreatefrompng( $source_path );
      break;
  }

  $source_aspect_ratio = $source_width / $source_height;
  $desired_aspect_ratio = DESIRED_IMAGE_WIDTH / DESIRED_IMAGE_HEIGHT;

  if ( $source_aspect_ratio > $desired_aspect_ratio )
  {
    //
    // Triggered when source image is wider
    //
    $temp_height = DESIRED_IMAGE_HEIGHT;
    $temp_width = ( int ) ( DESIRED_IMAGE_HEIGHT * $source_aspect_ratio );
  }
  else
  {
    //
    // Triggered otherwise (i.e. source image is similar or taller)
    //
    $temp_width = DESIRED_IMAGE_WIDTH;
    $temp_height = ( int ) ( DESIRED_IMAGE_WIDTH / $source_aspect_ratio );
  }

  //
  // Resize the image into a temporary GD image
  //

  $temp_gdim = imagecreatetruecolor( $temp_width, $temp_height );
  imagecopyresampled(
    $temp_gdim,
    $source_gdim,
    0, 0,
    0, 0,
    $temp_width, $temp_height,
    $source_width, $source_height
  );

  //
  // Copy cropped region from temporary image into the desired GD image
  //

  $x0 = ( $temp_width - DESIRED_IMAGE_WIDTH ) / 2;
  $y0 = ( $temp_height - DESIRED_IMAGE_HEIGHT ) / 2;

  $desired_gdim = imagecreatetruecolor( DESIRED_IMAGE_WIDTH, DESIRED_IMAGE_HEIGHT );
  imagecopy(
    $desired_gdim,
    $temp_gdim,
    0, 0,
    $x0, $y0,
    DESIRED_IMAGE_WIDTH, DESIRED_IMAGE_HEIGHT
  );

  //
  // Render the image
  // Alternatively, you can save the image in file-system or database
  //

  header( 'Content-type: image/jpeg' );
  imagejpeg( $desired_gdim, "image_files/".'$timestamp'.$_FILES["thumb"]["name"] );
  
  
  
mysql_connect("localhost", "root", "", "") or die(mysql_error());
mysql_select_db("") or die(mysql_error());

$timestamp = time();
$item_image = '"$timestamp"'.$_FILES['thumb']['name'];
  
$sql="UPDATE product SET thumb='$item_image' WHERE id = '$item_id'";

 if (!mysql_query($sql))

  {

  die('Error: ' . mysql_error());