# PHP | Check if a number is prime

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
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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:

 ` `

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:

 ` `

Output:

```Prime
```

Time Complexity: O(sqrt(n))

My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.

Article Tags :
Practice Tags :

1

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.