This cannot be done with GD, but I found a great library for it. This is a bit complicated, therefore, here is a link to a library that makes animated gifs with php. It explains how to use it fully. http://www.phpclasses.org/package/3163-PHP-Generate-GIF-animations-from-a-set-of-GIF-images.html
You can see my example using this library:
Just select 2 images and write 100 for a speed of 900 for width and height. He puts them in an animated gif slideshow.
Here is the code for this script:
<?php if(isset($_POST['speed'])) { header('Content-type: image/gif'); if(isset($_POST['download'])){ header('Content-Disposition: attachment; filename="animated.gif"'); } include('GIFEncoder.class.php'); function frame($image){ ob_start(); imagegif($image); global $frames, $framed; $frames[]=ob_get_contents(); $framed[]=$_POST['speed']; ob_end_clean(); } foreach ($_FILES["images"]["error"] as $key => $error) { if ($error == UPLOAD_ERR_OK) { $tmp_name = $_FILES["images"]["tmp_name"][$key]; $im = imagecreatefromstring(file_get_contents($tmp_name)); $resized = imagecreatetruecolor($_POST['width'],$_POST['height']); imagecopyresized($resized, $im, 0, 0, 0, 0, $_POST['width'], $_POST['height'], imagesx($im), imagesy($im)); frame($resized); } } $gif = new GIFEncoder($frames,$framed,0,2,0,0,0,'bin'); echo $gif->GetAnimation(); } ?> <form action="" method="post" enctype="multipart/form-data"> <script src="http://code.jquery.com/jquery-latest.js"></script> <script src="jquery.MultiFile.js"></script> <script src="jquery.placeholder.js"></script> <input type="file" name="images[]" class="multi" /> <script> $(function(){ $('input[placeholder], textarea[placeholder]').placeholder(); }); </script> <SCRIPT language=Javascript> <!-- function isNumberKey(evt) { var charCode = (evt.which) ? evt.which : event.keyCode if (charCode > 31 && (charCode < 48 || charCode > 57)) return false; return true; } </SCRIPT> <input name="speed" maxlength="10" type="text" placeholder="Speed of frames in ms" onkeypress="return isNumberKey(event)"> <input name="width" maxlength="4" type="text" placeholder="Width" onkeypress="return isNumberKey(event)"> <input name="height" maxlength="4" type="text" placeholder="Height" onkeypress="return isNumberKey(event)"> <input type="submit" name="download" value="Download!"> <input type="submit" name="preview" value="Preview!"> </form>
As you can see, it refers to the GIFEncoder class found at the first link. It also uses some javascript validation and multiupload jQuery.
By the way, this question has already been asked.