Variable Keys in PHP

Suppose we have the following array:

<?php
	$arr = ['a', 'b', 'c'];
?>

Let's output the element with key 0:

<?php
	$arr = ['a', 'b', 'c'];
	echo $arr[0]; // outputs 'a'
?>

Now, let's not write the key of the element to output directly in the square brackets, but store it in a variable:

<?php
	$arr = ['a', 'b', 'c'];
	$key = 0; // store the key in a variable
?>

Now let's use our variable to output the corresponding element:

<?php
	$arr = ['a', 'b', 'c'];
	$key = 0; // store the key in a variable
	
	echo $arr[$key]; // outputs 'a'
?>

Given the following variables:

<?php
	$arr = [1, 2, 3, 4, 5];
	$key1 = 1;
	$key2 = 2;
?>

Find the sum of the elements whose keys are stored in the variables.

Given the following variables:

<?php
	$arr = ['a' => 1, 'b' => 2, 'c' => 3];
	$key = 'b';
?>

Output the array element whose key is stored in the variable $key.