<?php
    
    $gid = 0;
    $array = [ 1,1,1,3,3,3,4,4,5,1,1,1,3,3,4,4,2,2 ];
    
    foreach($array as $key => $value){
        
        if($key && $array[$key-1] != $value){
            $gid++; 
        }


        
        $array2[$gid][] = $value;
    }
    
    print_r($array2);