# How To Check If a Number is Prime using PHP

In PHP we can easily Check If a Number is Prime or not. Five simple examples of PHP program are given below to check if given number is prime or not.

## What is PHP Number

Prime number is a number which only has two factors, 1 and the number itself.

## Example 1

In this example a Modulo operator is used to check the remainder of 10 divided by all the numbers from 1 to 10. If remainder comes 0 more than 3 times then it means the number (10 in this example) is not a prime number.

``` <?php \$number = 10; \$count=0; for ( \$i=1; \$i <= \$number; \$i++) { if ((\$number % \$i)==0) { \$count++; } } if (\$count < 3) { echo "\$number is a prime number."; } else { echo "\$number is not a prime number."; } ?> ```

### Output

10 is not a prime number.

## Example 2

This example also works on same principle, but in this example a function is created to check the if number is prime or not.

``` <?php function PrimeNumber(\$n) { for(\$a=2; \$a < \$n; \$a++) { if(\$n % \$a == 0) { return 0; } } return 1; } \$x = PrimeNumber(5); if (\$x == 0) { echo "This is not a Prime Number."; } else { echo "This is a Prime Number."; } ?> ```

### Output

This is a Prime Number.

## Example 3

gmp_prob_prime() function can also be used to check if a number is prime or not. If the function returns 0 it means that the number is not prime, if it returns 1 then number is probably prime, but if it returns 2 then number is surely a prime number.

``` <?php echo gmp_prob_prime("6"); ?> ```

0

## Example 4

In this example a number is divided by 2 to n/2 and remainder is checked every time, if the number divides completely then 0 will be returned, meaning that the number given is not a prime number.

``` <?php function primeCheck(\$y){ \$num = \$y; if (\$num == 1) { return 0; } for (\$i = 2; \$i <= \$num/2; \$i++){ if (\$num % \$i == 0) return 0; } return 1; } \$num = 27; \$x = primeCheck(\$num); if (\$x == 1) { echo "Prime Number"; } else { echo "Not a Prime Number"; } ?> ```

### Output

Not a Prime Number

## Example 5

The following example is suitable for larger numbers since it's much more efficient. The example checks only till square root of n instead of n, Since if given number is divisible by square root of n then it must be divisible by the number n.

``` <?php function primeNumber(\$number){ if (\$number == 1) { return 0; } else { for (\$i = 2; \$i <= sqrt(\$number); \$i++){ if (\$number % \$i == 0) return 0; } return 1; } } \$num = 27; \$x = primeNumber(\$num); if (\$x == 1) { echo "Prime Number"; } else { echo "Not a Prime Number"; } ?> ```

### Output

Not a Prime Number