`1` ```exercism fetch php luhn ```

# Luhn

Given a number determine whether or not it is valid per the Luhn formula.

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

## Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

## Example 1: valid credit card number

 `1` ```4539 1488 0343 6467 ```

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

 `1` ```4_3_ 1_8_ 0_4_ 6_6_ ```

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

 `1` ```8569 2478 0383 3437 ```

Then sum all of the digits:

 `1` ```8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80 ```

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

## Example 2: invalid credit card number

 `1` ```8273 1232 7352 0569 ```

Double the second digits, starting from the right

 `1` ```7253 2262 5312 0539 ```

Sum the digits

 `1` ```7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57 ```

57 is not evenly divisible by 10, so this number is not valid.

## Running the tests

1. Go to the root of your PHP exercise directory, which is `<EXERCISM_WORKSPACE>/php`. To find the Exercism workspace run

 `1` ```% exercism debug | grep Workspace ```
2. Get PHPUnit if you don't have it already.

 ```1 2``` ```% wget --no-check-certificate https://phar.phpunit.de/phpunit.phar % chmod +x phpunit.phar ```
3. Execute the tests:

 `1` ```% ./phpunit.phar luhn/luhn_test.php ```

## Source

The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.