Eliminate PHP strict warnings from nested array function calls

Problem/Motivation

A PHP strict warning is emitted by the following code due to use of the return value of the nested function call as the parameter to array_pop or array_shift, as opposed to a variable. The warning is: "Only variables should be passed by reference."

$default_pivot_col = empty($this->options['pivot_col']) ?
  array_pop(array_keys($columns)) :
  $this->options['pivot_col'];
$default_pivot_row = empty($this->options['pivot_row']) ?
  array_shift(array_keys($columns)) :
  $this->options['pivot_row'];

Proposed resolution

Create a variable for the column keys to pass to the outer array function calls.

Remaining tasks

Review and test. No documentation.

User interface changes

None.

API changes

None.