Description:
------------
According to the documentation at http://php.net/manual/en/function.array-keys.php, the default value for the second parameter in array_keys ($search_value) is null.  However, if you call the function and pass in null as the second value, the function returns an empty array, instead of the array of keys (as it appears to be searching for actual keys that match null).  Shouldn't a function behave the same way if a parameter is left blank or if the default value is passed in?

Test script:
---------------
<?php

$muppetColors = [
    'fozzie' => 'orange',
    'kermit' => 'green'
];

print_r(array_keys($muppetColors));

print_r(array_keys($muppetColors, null));

Expected result:
----------------
Both print_r statements should return arrays [ 'fozzie', 'kermit' ].

Actual result:
--------------
The first print_r statement returns the expected array [ 'fozzie', 'kermit' ], but the second print_r returns an empty array.