Checking the Existence of an Array Element in PHP

Using isset you can check the existence of an array element:

php<?php
  $arr = ['a', 'b', 'c'];
  
  if (isset($arr[5])) {
    echo $arr[5];
  } else {
    echo 'element does not exist';
  }
?>

You can also check an element of a non-existent array:

php<?php
  if (isset($arr[5])) {
    echo $arr[5];
  } else {
    echo 'array or element does not exist';
  }
?>

Задание 1

Without running the code, determine what will be displayed on the screen:

php<?php
  $test = 0;
  
  if (isset($test)) {
    echo '+++';
  } else {
    echo '---';
  }
?>

Задание 2

Without running the code, determine what will be displayed on the screen:

php<?php
  $test = null;
  
  if (!isset($test)) {
    echo '+++';
  } else {
    echo '---';
  }
?>

Задание 3

Without running the code, determine what will be displayed on the screen:

php<?php
  $test = null;
  
  if (isset($test)) {
    echo '+++';
  } else {
    echo '---';
  }
?>

Задание 4

Without running the code, determine what will be displayed on the screen:

php<?php
  if (!isset($test)) {
    echo '+++';
  } else {
    echo '---';
  }
?>

Задание 5

Without running the code, determine what will be displayed on the screen:

php<?php
  $test = '';
  
  if (isset($test)) {
    echo '+++';
  } else {
    echo '---';
  }
?>

Задание 6

Without running the code, determine what will be displayed on the screen:

php<?php
  if (!isset($test)) {
    echo '+++';
  } else {
    echo '---';
  }
?>

Задание 7

Without running the code, determine what will be displayed on the screen:

php<?php
  $test = false;
  
  if (isset($test)) {
    echo '+++';
  } else {
    echo '---';
  }
?>

Задание 8

Without running the code, determine what will be displayed on the screen:

php<?php
  $arr = [1, 2, 3, 4, 5];
  
  if (isset($arr[])) {
    echo '+++';
  } else {
    echo '---';
  }
?>