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# Binary
Convert a binary number, represented as a string (e.g. '101010'), to its decimal equivalent using first principles.
Implement binary to decimal conversion. Given a binary input
string, your program should produce a decimal output. The
program should handle invalid inputs.
## Note
- Implement the conversion yourself.
Do not use something else to perform the conversion for you.
## About Binary (Base-2)
Decimal is a base-10 system.
A number 23 in base 10 notation can be understood
as a linear combination of powers of 10:
- The rightmost digit gets multiplied by 10^0 = 1
- The next number gets multiplied by 10^1 = 10
- ...
- The *n*th number gets multiplied by 10^*(n-1)*.
- All these values are summed.
So: `23 => 2*10^1 + 3*10^0 => 2*10 + 3*1 = 23 base 10`
Binary is similar, but uses powers of 2 rather than powers of 10.
So: `101 => 1*2^2 + 0*2^1 + 1*2^0 => 1*4 + 0*2 + 1*1 => 4 + 1 => 5 base 10`.
## Running the tests
1. Go to the root of your PHP exercise directory, which is `<EXERCISM_WORKSPACE>/php`.
To find the Exercism workspace run
% exercism debug | grep Workspace
1. Get [PHPUnit] if you don't have it already.
% wget --no-check-certificate https://phar.phpunit.de/phpunit.phar
% chmod +x phpunit.phar
2. Execute the tests:
% ./phpunit.phar binary/binary_test.php
[PHPUnit]: http://phpunit.de
## Source
All of Computer Science [http://www.wolframalpha.com/input/?i=binary&a=*C.binary-_*MathWorld-](http://www.wolframalpha.com/input/?i=binary&a=*C.binary-_*MathWorld-)
## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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