How to Check Whether a Number is Prime in PHP

Imagine a given number and you want to check whether it is prime or not with PHP.

Below, we will represent two methods: a simple method and a more efficient one.

Read on to explore them.

Simple Solution

Let’s start with a simpler solution. It considers iterating through all the numbers from 2 to n / 2 for each number checking whether it divides n evenly.

After finding any number that divides, false should be returned, otherwise, true.

The implementation of this solution is below:

php check number is prime or not

<?php

// PHP code for checking if a number is prime or not
// function for checking the number is Prime or not
function primeCheck($number)
{
    if ($number < 2) {
        return false;
    }
    for ($i = 2; $i <= $number / 2; $i++) {
        if ($number % $i == 0) {
            return false;
        }
    }
    return true;
}

// Driver Code
$number = 31;
if (primeCheck($number)) {
    echo "Prime";
} else {
    echo "Not Prime";
}

The output of the code is as follows:

Prime

A Complex and Efficient Method

Now, let’s explore a more complicated but much more efficient solution.

To implement that solution, it is necessary to optimize the first approach by checking sqrt(n) instead of n. The reason is that a larger factor of n should be multiple of a smaller factor, which has been checked.

The demonstration of this solution is below:

php code to check if a number is prime

<?php

// PHP code for checking if a number is prime or Not
// function for checking if the number is Prime or Not
function primeCheck($number)
{
  if ($number < 2) {
    return false;
  }
  if ($number == 2) {
    return true;
  }
  if ($number % 2 == 0) {
    return false;
  }

  for ($i = 3; $i <= sqrt($number); $i += 2) {
    if ($number % $i == 0) {
      return false;
    }
  }
  return true;
}

// Driver Code
$number = 31;

if (primeCheck($number)) {
  echo "Prime";
} else {
  echo "Not Prime";
}

Prime

Defining prime Numbers

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. The underlying logic for checking primality remains the same across programming languages; only the syntax changes.