How to put GIF images together

I have one arrays with GIF pictures (transparent).

It's a layer up to 11 pictures.
Now I like to put all the layers together in one picture.
Could somebody tell me if they are a library who can do
that ?

Example:
echo "<br>The pictures : ";
for ($i = 1;$i <= 11; $i++) {
$img = $_SESSION['tableau'][$i][2];
// echo for debug
echo "<br>$i.-.$img";
// if this layer have a picture
if($img){
..
$pict = $pict + ...... // Or something like that!
..
}
}

Many thanks in advance for your help.

Best regards

Otto