# 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))