It s not possible using the standard GD functions that come pre-packed with PHP.

There is a class on phpclasses.org for this. I have never used it myself, but it s used by a lot of other packages.

Alternatively, if you have access to ImageMagick from PHP, using either the MagickWand library or the command line, use it. With ImageMagick, it s no problem.

• ImageMagick v6 Animation basics (from the IM manual)

• Creating an Animated GIF image

For a nice, quick and more recent solution, see this SO answer.

For an even more recent solution, here is my fork of it, with a number of small fixes & improvements. An example of it from an actual application:

$anim = new GifCreatorAnimGif();

$gif = $anim->create($image_files);
//file_put_contents("test.gif", $gif);

header("Content-type: image/gif");
echo $gif;

(Requires PHP5.3 with GD2.)

Example that works with PHP 5.6 and GD 2.4.11:

require_once "AnimGif.php";

/*
 * Create an array containing file paths, resource var (initialized with imagecreatefromXXX), 
 * image URLs or even binary code from image files.
 * All sorted in order to appear.
 */
$image_files = array(
    //imagecreatefrompng("/../images/pic1.png"), // Resource var
    //"/../images/pic2.png", // Image file path
    //file_get_contents("/../images/pic3.jpg"), // Binary source code
     https://yt3.ggpht.com/-KxeE9Hu93eE/AAAAAAAAAAI/AAAAAAAAAAA/D-DB1Umuimk/s100-c-k-no-mo-rj-c0xffffff/photo.jpg , // URL
     https://media.licdn.com/mpr/mpr/shrinknp_100_100/AAEAAQAAAAAAAAloAAAAJDRkZGY2MWZmLTM1NDYtNDBhOS04MjYwLWNkM2UzYjdiZGZmMA.png , // URL
     http://is5.mzstatic.com/image/thumb/Purple128/v4/e4/63/e7/e463e779-e6d0-0c3d-3ec1-97fdbaae230a/source/100x100bb.jpg  // URL
);

/*
 * Create an array containing the duration (in millisecond) of each frame.
 */
$durations_millis = array(
    1000,
    2000,
    3000
);

/*
 * Fix durations.
 */
$durations = array();
for ($i = 0; $i < count($durations_millis); $i++) {
    $durations[$i] = $durations_millis[$i] / 10;
}

/*
 * Specify number of loops. (0 = infinite looping.)
 */
$num_loops = 0;

/*
 * Create gif object.
 */
$anim_gif = new GifCreatorAnimGif();
$gif_object = $anim_gif->create($image_files, $durations, $num_loops);

/*
 * Get the animated GIF binary.
 */
$gif_binary = $gif_object->get();

/*
 * Set the file name of the saved/returned animated GIF.
 */
$file_name = "animated.gif";

/*
 *  Optionally, save animated GIF in a folder as a GIF:
 */
//file_put_contents($file_name, $gif_binary);

/*
 * Optionally, return the animated GIF to client.
 */
header("Content-type: image/gif");
header( Content-Disposition: filename="  . $file_name .  " ); // Optional
echo $gif_binary;

/*
 * All done.
 */
exit;

This cannot be done with GD but I found a great library for it. It is a bit complicated though, so here is a link to the library which makes animated gifs with php. It explains how to use it thoroughly. http://www.phpclasses.org/package/3163-PHP-Generate-GIF-animations-from-a-set-of-GIF-images.html

Select 2 pictures and write 100 for speed 900 for width and height. It will put them in an animated gif slideshow.

Here is the code for that 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 see it references the GIFEncoder class found on the first link. It also uses some javascript validation and jQuery multiupload.

Note: this question has already been asked.