SwapRowsAndColumns.php

<?php
    /**
     * @param array[] $matrix
     * @param string[] $fields
     * @return array[]
     */
    public function tilt(array $matrix, array $fields = array())
    {
        $topKeys = array_keys($matrix);
        $nestedKeys = $fields ?: array_keys((array)current($matrix));
        $numResults = count($matrix);
        $matrix = array_pad($matrix, 2, array_fill_keys($nestedKeys, null)); //Empty sets still need to result in arrays
        array_unshift($matrix, null);   //Sending null as the array_map() closure is what tilts the matrix
        $matrix = call_user_func_array('array_map', $matrix);
        $matrix = $nestedKeys ? array_combine($nestedKeys, $matrix) : array();  //Preserve and move the keys
        array_walk($matrix, function (array &$column) use ($numResults, $topKeys) {
            $column = array_slice($column, 0, $numResults); //Prune off the placeholders, if any were needed
            $column = array_combine($topKeys, $column);
        });

        return $matrix;
    }