<?php
 
$verticies =
[
    [0, 0, 0],
    [1, 0, 0],
    [1, 1, 1],
    [0, 1, 1],
];  
 
 
for ($i = 0; $i < count($verticies); $i++) 
{
    //current vertex
    $pi = $verticies[$i];
 
    //next vertex
    $pj = $verticies[($i+1) % count($verticies)];
 
    //0 = x, 1 = y, 2 = z
    $nx += ((($pi[2]) + ($pj[2])) * (($pj[1]) - ($pi[1])));
    $ny += ((($pi[0]) + ($pj[0])) * (($pj[2]) - ($pi[2])));
    $nz += ((($pi[1]) + ($pj[1])) * (($pj[0]) - ($pi[0])));
}
 
echo $nx.', '.$ny.', '.$nz;

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