How to check number is prime or not?

Below program checks whether a given number is prime or not:

<?php

// function will check number is prime of not and return true or false
function isPrimeNumber($number) 
{
    if ($number <= 1) {
        return false;
    }

    for ($i = 2; $i <= sqrt($number); $i++) {
        if ($number % $i == 0) {
            return false;
        }
    }

    return true;
}

$number = 19;

echo "Output:<br> $number is " . ((isPrimeNumber($number))?"":"not") . " a prime number.";

$number = 33;

echo "<br>Output:<br> $number is " . ((isPrimeNumber($number))?"":"not") . " a prime number.";

Results: