PHP Examples
Python Java C++ C C# PHP R SQL DS Algo InterviewQ

PHP Program to Check Prime Number



A Prime number is a natural number greater than 1 and divisible by 1 and itself only, for example: 2, 3, 5, 7, etc.

Method 1: Using conditional statements

In the example below, the number called MyNum is checked for prime number by dividing it with all natural numbers starting from 2 to N - 1.

<?php
$MyNum = 17;
$n = 0;

for($i = 2; $i < $MyNum; $i++) {
  if($MyNum % $i == 0){
    $n++;
    break;
  }
}

if ($n == 0){
  echo $MyNum." is a prime number.";
} else {
  echo $MyNum." is not a prime number.";
}
?>

The above code will give the following output:

17 is a prime number.

Method 2: Using function

In the example below, a function called primenumber() is created which takes a number as argument and checks it for prime number by dividing it with all natural numbers starting from 2 to N/2.

<?php
function primenumber($MyNum) {
  $n = 0;

  for($i = 2; $i < ($MyNum/2+1); $i++) {
    if($MyNum % $i == 0){
      $n++;
      break;
    }
  }

  if ($n == 0){
    echo $MyNum." is a prime number.";
  } else {
    echo $MyNum." is not a prime number.";
  }
}

primenumber(21);
?>

The above code will give the following output:

21 is not a prime number.

Method 3: Optimized Code

  • Instead of checking the divisibility of given number from 2 to N/2, it is checked till square root of N. For a factor larger than square root of N, there must the a smaller factor which is already checked in the range of 2 to square root of N.
  • Except from 2 and 3, every prime number can be represented into 6k ± 1.
<?php
function primenumber($MyNum) {
  $n = 0;
  if ($MyNum == 2 || $MyNum == 3) {
    echo $MyNum." is a prime number.";
  } elseif ($MyNum % 6 == 1 || $MyNum % 6 == 5) {
      for($i = 2; $i*$i <= $MyNum; $i++) {
        if($MyNum % $i == 0){
          $n++;
          break;
        }
      }

      if ($n == 0){
        echo $MyNum." is a prime number.";
      } else {
        echo $MyNum." is not a prime number.";
      }
  } else {
    echo $MyNum." is not a prime number.";
  }
}

primenumber(21);
?>

The above code will give the following output:

21 is not a prime number.




5