Multidimensional Arrays in PHP

Array elements can be not only strings and numbers, but also arrays. In this case, we get an array of arrays or a multidimensional array. In the following example, the array $arr consists of three elements, which in turn are arrays:

<?php
	$arr = [['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i']];
?>

Let's rewrite it in a more understandable form:

<?php
	$arr = [
		['a', 'b', 'c'],
		['d', 'e', 'f'],
		['g', 'h', 'i'],
	];
?>

Depending on the nesting level, arrays can be two-dimensional - an array of arrays, three-dimensional - an array of arrays of arrays (and so on - four-dimensional, five-dimensional, etc.).

The array above is two-dimensional, since inside one array there are other subarrays and these subarrays do not contain other arrays. To output any element from a two-dimensional array, you should write not one pair of square brackets, but two:

<?php
	$arr = [
		['a', 'b', 'c'],
		['d', 'e', 'f'],
		['g', 'h', 'i'],
	];
	
	echo $arr[0][1]; // outputs 'b'
	echo $arr[1][2]; // outputs 'f'
?>

Given the following array:

<?php
	$arr = [
		['a', 'b', 'c'],
		['d', 'e', 'f'],
		['g', 'h', 'i'],
		['j', 'k', 'l'],
	];
?>

Using it, output the elements with the text 'l', 'e', 'g' and 'a'.

Given the following array:

<?php
	$arr = [[1, 2], [3, 4], [5, 6]];
?>

Find the sum of all its elements.