Write a function named remove_consecutive_duplicates that accepts as a parameter
an array of numbers, and modifies it by removing any
consecutive duplicates.
For example, if an array named $arr stores [1, 2, 2, 3, 2, 2, 3], the call of
remove_consecutive_duplicates($arr) should modify it to store [1, 2, 3, 2, 3].
You may assume that the array passed is non-null, but it may be empty (in which case it would
trivially contain no duplicates and should be left unchanged).
A note about references in PHP: In order to write a function passes a parameter as
reference (thus modifying its state), you'll need to prepend "&" to the variable declaration
in the function header. For example, a function foo that modifies the state
of an array parameter may be defined as:
function foo(&$arr) { ... }
