- array_splice: documentation ( source)
- print_r: documentation ( source)
<?php
$listallfriendsarray = [1,2,3,4,5,6,7];
$listalreadydisplayed = [1,3,5,6];
array_splice($listallfriendsarray,0,count($listalreadydisplayed),$listalreadydisplayed);
print_r($listallfriendsarray);