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); 

?>