In mathematics, one of the definitions of odd and even number is that they can be expressed like this (where n is the number and k is an integer):

Odd numbers:n = 2k +1

Even numbers:n = 2k

In PHP, we can check a number’s parity by checking the remainider of the division of this number by 2 using the modulus operator (%), if the remainder is 1, the number is odd, if the remainder is 0, the number is even, let’s write some functions for this:

<?php function is_odd($n){ return (boolean) ($n % 2); } function is_even($n){ if(is_odd($n)) return FALSE; return TRUE; } $a = 1; $b = 2; $c = 3; echo "$a is ", is_odd($a) ? 'odd' : 'even' , '.<br />'; echo "$b is ", is_odd($b) ? 'odd' : 'even' , '.<br />'; echo "$c is ", is_even($c)? 'even' : 'odd' , '.<br />'; ?>

The output in the browser will look like:

1 is odd.

2 is even.

3 is odd.

This is a simple way to do it, I used typecasting in the function *is_odd()* so it’s always return TRUE or FALSE, instead of 1 or 0.

#### PHP modulus (%)

Operands of modulus are converted to integers (by stripping the decimal part) before processing.

<?php echo 1 % 2; echo 1.01 % 2; // each of these three statements outputs 1. echo 1.99 % 2; ?>