PHP Program to Find All Prime Numbers in a Given Interval

Prime numbers are fundamental in the field of mathematics and computer science. A prime number is defined as a natural number greater than 1 and is divisible by only 1 and itself.

In this article, we will explore how to write a PHP program to find all prime numbers within a given interval.

Using Trial Division Method

The trial division method is a straightforward way to check whether a number is prime. We iterate through all numbers in the given interval and test each number for primality.

PHP

<?php

function isPrime($num) {
    if ($num < 2) {
        return false;
    }
    for ($i = 2; $i <= sqrt($num); $i++) {
        if ($num % $i == 0) {
            return false;
        }
    }
    return true;
}

function findPrimeNums($start, $end) {
    $primes = [];
    for ($i = $start; $i <= $end; $i++) {
        if (isPrime($i)) {
            $primes[] = $i;
        }
    }
    return $primes;
}

$start = 10;
$end = 50;
$primeNumbers = findPrimeNums($start, $end);

echo implode(', ', $primeNumbers);

?>

Output

11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47

Using Sieve of Eratosthenes

The Sieve of Eratosthenes is an ancient algorithm for finding all prime numbers to a given limit. It works by iteratively marking the multiples of each prime, starting from 2.

PHP

<?php

function findPrimeNums($n) {
    $isPrime = array_fill(2, $n, true);
    for ($i = 2; $i * $i <= $n; $i++) {
        if ($isPrime[$i]) {
            for ($j = $i * $i; $j <= $n; $j += $i) {
                $isPrime[$j] = false;
            }
        }
    }

    return array_keys(array_filter($isPrime));
}

// Driver code
$start = 10;
$end = 50;

$primeNums = findPrimeNums($end);

$primeNumsInRange = array_filter($primeNums,
    function ($num) use ($start, $end) {
        return $num >= $start && $num <= $end;
    });

echo implode(', ', $primeNumsInRange);

?>

Output

11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47