Compress JSON Array in PHP

/**
 * Converts a list of associative arrays into indexed arrays.
 *
 * Each property name is collated within an extra array inserted at the beginning
 * of the returned array, structuring the data in a CSV-like fashion. For example:
 *
 *   array(
 *       array('first_name' => 'John',   'last_name' => 'Gardner',   'age' => 28),
 *       array('first_name' => 'Sum',    'last_name' => 'Kunt',      'age' => 25)
 *   )
 *
 * Would be returned as:
 *
 *   array(
 *       array('first_name', 'last_name',    'age'),
 *       array('John',       'Gardner',      28),
 *       array('Sum',        'Kunt',         25)
 *   )
 *
 * Properties are indexed in the order they're encountered when traversing the original data
 * list, with any missing properties assigned empty values if they're present in some rows
 * but absent in others. For instance, consider the following inconsistent data structure,
 * with two rows sharing only partial similarity:
 *
 *   array(
 *       array('first_name' => 'John',   'age' =>  28),
 *       array('first_name' => 'Sum',    'dob' => '1990-08-20')
 *   )
 *
 * These would be collated and returned as:
 *
 *   array(
 *       array('first_name',   'age',    'dob'),
 *       array('John',         '28',      NULL),
 *       array('Sum',           NULL,    '1990-08-20')
 *   )
 *
 * This ensures data will be ordered in a consistent format, and can help reduce latency
 * and bandwidth costs when sending large payloads of JSON-encoded data.
 *
 * @param array $data - A multidimensional array to operate upon
 * @return array
 */
function collate_arrays($data){
    $output     =   array();

    # First, cycle through each row to extract a list of every unique field name that's present.
    $fields     =   array();
    $count      =   0;

    foreach($data as $row){
        foreach($row as $name => $value)
            if(!isset($fields[$name]))
                $fields[$name]  =   $count++;
    }


    # Flip the list of field names and store them in the first row of the returned array.
    $fields     =   array_flip($fields);
    $output[]   =   $fields;


    # Next, run through the array again, ensuring every array and its properties are consistently indexed in running field order.
    foreach($data as $row){
        $indexed_row = array();

        for($i = 0; $i < $count; ++$i)
            $indexed_row[$i] = $row[$fields[$i]];

        $output[]   =   $indexed_row;
    }

    return $output;
}