Stack abstract data structure

stack.php

<?php

function push($item = null)
{
    $stack = [];
    if ($item) {
        $stack[] = $item;
    }
    return $stack;
}

function pop($stack)
{
    if (count($stack) > 0) {
        $size = count($stack);
        unset($stack[$size - 1]);
    }
    return $stack;
}
    

$stack[] = push('a');
$stack[] = push('b');
$stack[] = push('c');
$stack[] = push('d');

$stack = pop($stack);
$stack = pop($stack);
$stack = pop($stack);

var_dump($stack);