to clear all piece in board, we simply call clear()
<?php
use \Ryanhs\Chess\Chess;
$chess = new Chess();
$chess->clear();
echo 'FEN => ' . $chess->generateFen() . PHP_EOL;
echo $chess;
will return
FEN => 8/8/8/8/8/8/8/8 w - - 0 1
+------------------------+
8 | . . . . . . . . |
7 | . . . . . . . . |
6 | . . . . . . . . |
5 | . . . . . . . . |
4 | . . . . . . . . |
3 | . . . . . . . . |
2 | . . . . . . . . |
1 | . . . . . . . . |
+------------------------+
a b c d e f g h