is_Prime() function on PHP

All primes end in either 1, 3, 7, or 9. You could do a check up front to make sure the number satisfies that requirement.

Second, a number is prime if it has no prime factors besides itself and 1. Therefore you should start you checking loop at 3, not at 1. Since you only need to check against primes, you could start the loop at 3, increment it by 10, and check $x/$i, $x/($i+4), $x/($i+6), and $x/($i+8). That would be 4 divide operations instead of the 10 you are presently doing.

Third, you only have to check possible factors up to the square root of the number you are checking, so figure out the sqrt($x) and make that the end of the loop. You are also putting all the factors you find into an array, which is not necessary if you are simply trying to see if the number is prime.

Here's my version of your code and an explanation of what I did and why.

<?php

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

    $isPrime = true;
    for ($i = 2; $i < $number ; $i++) {
        if (is_integer($number / $i)) {
            $isPrime = false;
            break;
        }
    }

    return $isPrime;
}

for ($i = 0; $i <= 100; $i++) {
    echo $i;
    echo isPrime($i) ? " is a prime number" : " isn't a prime number";
    echo "<br>";
}

?>

1. Use four spaces for indentation as per PSR-2. This will make your code more readable.
2. Stick to camelCase for your function name as per PSR-1. This will make your code more readable.
3. Make your function return true or false so you can use your code in multiple places without having to change what it outputs. This will make your code reusable.
4. Make your function properties more descriptive. This will make your code more readable.
5. Make your for loop start at two as we have establish at the top of the function that the number isn't equal to one or less. This will increase performance of your code.
6. Make your loop stop at one number before the number in question as we know it will be divisible by itself. This will increase performance of your code.
7. Make your loop exit as soon as it hits an integer. We have already excluded the number itself and the number one so we can be sure it isn't a prime number. This will increase performance of your code.