PHP Program to Check Prime Number


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

Example: Using conditional statements

In the below example, 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.";
  }
?>

Output

17 is a prime number.

Example: Using function

In the below example, 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 $$\frac{N}{2}$$.

<?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);
?>

Output

21 is not a prime number.

Example: Optimized Code

  • Instead of checking the divisibility of given number from $$2$$ to $$\frac{N}{2}$$, it is checked till $$\sqrt{N}$$. For a factor larger than $$\sqrt{N}$$, there must the a smaller factor which is already checked in the range of $$2$$ to $$\sqrt{N}$$.
  • Except from $$2$$ and $$3$$, every prime number can be represented into $$6k\pm1$$.

<?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);
?>

Output

21 is not a prime number.




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