Add GD image inside another GD image

How can I embed a dynamic image created with GD inside another dynamic image generated with GD? Both files work fine if they are separated, but when I try to use one inside the another it doesn't work.

Image 1:

<?php
header('Content-type: image/png');
$image1 = imagecreatefrompng('images/image1.png');

//add some texts to image 1

imagepng($image1);

imagedestroy($image1);
?>

Image 2:

<?php
header('Content-type: image/png');
$image2 = imagecreatefrompng('images/image2.png');

//add some texts to image 2

imagepng($image2);

imagedestroy($image2);
?>

Now, if I try to use image generated in image2.php inside image1.php:

<?php
header('Content-type: image/png');
$image1 = imagecreatefrompng('images/image1.png');
$image2 = imagecreatefromgd('image2.php');

//add some texts to image1
imagecopy($image1, $image2, $x, $y, $src_x, $src_y, $src_w, $src_h);

imagepng($image1);

imagedestroy($image1);
imagedestroy($image2);
?>

Any ideas? Thanks!