<?php
$people = [
    ['name'=> 'Silvia' , 'height'=> 1.69],
    ['name'=> 'Martin' , 'height'=> 1.82],
    ['name'=> 'Pesho' , 'height'=> 1.55],
    ['name'=> 'Linda' , 'height'=> 1.88],
];
$averageHeight = array_reduce ($people, function ($carry, $item) {
        return $carry + $item['height'];});

echo $averageHeight / count($people);