Removed by ref method call in decode()

A by ref call on a method throws an error under strict settings. `$header_byte = array_shift(unpack("C*", substr($var, 0, 1)));` should be changed to:

```php
$unpacked = unpack("C*", substr($var, 0, 1));
$header_byte = array_shift($unpacked);
```

This ensures that a variable is called by reference instead of a method. You can read more about this [here](http://stackoverflow.com/questions/9848295/strict-standards-only-variables-should-be-passed-by-reference-error) and [here](http://stackoverflow.com/questions/2354609/strict-standards-only-variables-should-be-passed-by-reference).