Imagick::writeImages

The second parameter spicifies if the object is write in multipage file (true) or split every page in a single file (false).
Example:
<?php
$multiTIFF = new Imagick();
$files = scandir($mytifspath);
 
foreach( $files as $f )
{
 $auxIMG = new Imagick();
 $auxIMG->readImage($mytifspath.$f);
 
 $multiTIFF->addImage($auxIMG);
}
//file multi.TIF
$multiTIFF->writeImages('multi.TIF', true);
//files multi-0.TIF, multi-1.TIF, ...
$multiTIFF->writeImages('multi.TIF', false);
?>

As mbrugue sugest, the second parameter can also be use to save an animated gif:
<?php 
$anim = new Imagick(); 
$files = scandir($myFramesPath); 
 
foreach( $files as $f ) 
{ 
 $auxIMG = new Imagick(); 
 $auxIMG->readImage($mytifspath.$f); 
 
 $anim->addImage($auxIMG); 
} 
//write animated gif
$anim->writeImages('anim.gif', true); 
?>