PHP | Check if a number is prime
Last Updated :
10 Jul, 2021
Given a number, we need to check whether it is prime or not in PHP. General approach for prime check is discussed here. In this article we will learn about how to check if a number is prime or not in PHP.
Examples:
Input : 21
Output : Not Prime
Input : 31
Output : Prime
Simple Method:
A simple solution is to iterate through all numbers from 2 to n/2 and for every number check if it divides n. If we find any number that divides, we return 0 (false) otherwise we will return 1 (true).
Below is the implementation of this approach in PHP:
PHP
<?php
function primeCheck($number){
if ($number == 1)
return 0;
for ($i = 2; $i <= $number/2; $i++){
if ($number % $i == 0)
return 0;
}
return 1;
}
$number = 31;
$flag = primeCheck($number);
if ($flag == 1)
echo "Prime";
else
echo "Not Prime"
?>
|
Output:
Prime
Time Complexity: O(n)
Efficient Method:
We can optimize the above approach by observing that, instead of checking till n, we can check till sqrt(n) because a larger factor of n must be a multiple of smaller factor that has been already checked.
So, we will traverse in the range [2,sqrt(number)] to check if the number is divisible by any number or not. If it is divisible the its not a prime number.
Below is the implementation of this approach in PHP:
PHP
<?php
function primeCheck($number){
if ($number == 1)
return 0;
for ($i = 2; $i <= sqrt($number); $i++){
if ($number % $i == 0)
return 0;
}
return 1;
}
$number = 31;
$flag = primeCheck($number);
if ($flag == 1)
echo "Prime";
else
echo "Not Prime"
?>
|
Output:
Prime
Time Complexity: O(sqrt(n))
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