## Prime Number Program in PHP using while loop and for loop

We have been trying to give more and more **PHP programming examples for beginners **here and today we are doing so as well. Today we will be going to give you *PHP code examples* for Prime Numbers. So without further ado let’s start the **Prime number program in PHP using if else.**

What is a Prime Number?

Before writing **program to find prime numbers in PHP **you have to now what are the Prime numbers or prime number definition. Here are three ways in which you can describe a prime number.

- A prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers.
- A number that is divisible only by itself and 1.
- A whole number that can
**not**be made by multiplying other smaller whole numbers.

Note: 2 is the only even number that is a prime or a prime number.

How to find a Prime Number?

- How to find a whole number is a prime number or not, take a square root of the given number. Test whether the number is divisible by any prime number less than the square root. If yes, than the given number is not a prime number. If not, than the number is a prime number.
- Other method is the factorization method as to factor a number is to split that number down into smaller parts. To find out the prime factorization of a number, you require to break the number down to its prime factors.

Now as we know that what a prime number is and how to check a number if it is a prime number or not, let’s start writing the PHP function for Prime Numbers.

function isPrimeNumber($num) { //1 is not a prime number</code> if($num == 1){ return false; } //2 is prime (the only even number that is prime) if($num == 2){ return true; } // No even number is a prime number other than 2 if($num % 2 == 0) { return false; } /* Checks the odd numbers. If any of them is a factor, then it returns false. The sqrt can be an aproximation, hence just for the sake of security, one rounds it to the next highest integer value. */ $ceil = ceil(sqrt($num)); for($i = 3; $i <= $ceil; $i = $i + 2) { if($num % $i == 0) return false; } return true; }

Here is another way to check if a number is a prime number with while loop in php, **prime number program in PHP using while loop**:

function is_prime($number) { return !preg_match('/^1?$|^(11+?)\1+$/x', str_repeat('1', $number)); //using preg_match to find prime numbers } $start = 2; // start here (2 is the first prime) $end = 1000; $i=$start; while ($i<=$end) { if (is_prime($i)) echo $i." <br />\n"; $i++; }

Here is another function to find a prime using for loop in php:

function primeNumbers($n){ for($i=1;$i<=$n;$i++){ //numbers to be checked as prime $counter = 0; for($j=1;$j<=$i;$j++){ //all divisible factors if($i % $j==0){ $counter++; } } //prime requires 2 rules ( divisible by 1 and divisible by itself) if($counter==2){ print $i." is Prime <br/>"; } } }

This is another very simple and easy way of finding Prime Numbers in PHP:

function isPrimeNumber($i) { $n = 2; while ($n < $i) { if ($i % $n) { $n++; continue; } return false; } return true; }

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