PHP: Negation

Alongside the conjunction (AND) and disjunction (OR) operators, we often use the negation operator, too. It changes the logical value to the opposite one. It's denoted by the unary operator ! in programming.

If there's a function that checks for even numbers, then negation will allow you to check for odd numbers:

<?php

function isEven($number)
{
    return $number % 2 === 0;
}

isEven(10);  // true
!isEven(10); // false

I.e., we can just put a ! to the left of the function call and get the opposite result. Negation is a powerful tool that allows you to concisely express the desired rules in your code without having to write new functions.

What if you write it like this !!isEven(10)? Suddenly, the code will work. Double negation in logic is equivalent to no negation at all.

<?php

isEven(10);   // true
!isEven(10);  // false
!!isEven(10); // true

Instructions

In this lesson, you'll need to implement the two functions isPalindrome() and isNotPalindrome()

1. The isPalindrome() function determines whether a word is a palindrome or not. A palindrome is a word that reads the same backwards as it does forwards.

   <?php
   
   isPalindrome('wow'); // true
   isPalindrome('mom'); // true
   isPalindrome('hexlet'); // false
   
   // You can pass upper and lower-case letters to the function
   // Therefore, you must first convert the word to lowercase strtolower()
   isPalindrome('Wow'); // true
   

   To flip a word, use the strrev() function.

2. The isNotPalindrome() function checks that the word is NOT a palindrome:

   <?php
   
   isNotPalindrome('wow'); // false
   isNotPalindrome('mom'); // false
   isNotPalindrome('hexlet'); // true
   

   All you need is to negate the isPalindrome() function call within the isNotPalindrome() function's body.

Tips

• De Morgan's Laws