Create a PHP script that checks whether a given number is a prime number or not.

<?php
function isPrime($number) {
    if ($number <= 1) {
        return false;
    }

    for ($i = 2; $i <= sqrt($number); $i++) {
        if ($number % $i === 0) {
            return false;
        }
    }

    return true;
}

$testNumber = 13;

if (isPrime($testNumber)) {
    echo "$testNumber is a prime number.";
} else {
    echo "$testNumber is not a prime number.";
}
?>

• The script defines a function isPrime that takes a number as a parameter and checks whether it is a prime number.
• A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers.
• The function uses a for loop to iterate from 2 to the square root of the number to check for divisors.
• If the number has any divisors other than 1 and itself, it is not a prime number.
• The script then calls the function with a sample number ($testNumber) and prints whether it is a prime number or not.