Accessing an Array Element in PHP

Accessing array elements is done similarly to accessing string characters. Let's look at an example:

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

Задание 1

Given an array:

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

Output each element of this array to the screen.

Задание 2

Given an array:

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

Using this array, output the following string:

php'a+b+c+d'

Задание 3

Given an array:

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

Output the sum of the array elements to the screen.

Задание 4

Given an array:

php<?php
  $arr = [2, 5, 3, 9];
?>

Multiply the first element of the array by the second, and the third element by the fourth. Add the results, assign them to the variable $res. Output the value of this variable to the screen.