Luhn in PHP
Given a number determine whether or not it is valid per the Luhn formula.
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 nondigit characters are disallowed.
Example 1: valid credit card number
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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
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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:
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8569 2478 0383 3437

Then sum all of the digits:
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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
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8273 1232 7352 0569

Double the second digits, starting from the right
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7253 2262 5312 0539

Sum the digits
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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

Go to the root of your PHP exercise directory, which is
<EXERCISM_WORKSPACE>/php
. To find the Exercism workspace run1
% exercism debug  grep Workspace

Get PHPUnit if you don't have it already.

Execute the tests:
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% ./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.