foldl.php

<?php

// [a] -> a
function head(array $list)
{
    return array_slice($list, 0, 1)[0];
}

// [a] -> a
function last(array $list)
{
    return array_slice($list, -1)[0];
}

// [a] -> [a]
function tail(array $list)
{
    return array_slice($list, 1);
}

// [a] -> [a]
function init(array $list)
{
    return array_slice($list, 0, -1);
}

// Foldable t => (b -> a -> b) -> b -> t a -> b
function foldl(callable $func, array $list, $initval)
{
    return [] === $list
        ? $initval
        : $func(
            foldl($func, init($list), $initval),
            last($list)
        );
}

// (a -> b) -> [a] -> [b] 
function map(callable $func, array $list): array
{
    return foldl(function (array $list, $n) use ($func) {
        return array_merge($list, [
            $func($n)
        ]);
    }, $list, []);
}

var_dump(
    map(
        function (int $x): int {
            return $x + 10;
        },
        [10, 20, 30]
    )
);

var_dump(
    foldl(
        function (int $x, int $y): int {
            return $x + $y;
        },
        [5, 10, 20],
        0
    )
);