php for 10 for echo substr -3

<?php
$s = 0;
for($i=1;$i<=10;$i++)
{
	$f = 1;
	for($n=2;$n<=$i;$n++)
		$f *= $n;
	$r .= " $f + ";
	$s += $f;
}
echo substr($r, 0, -3 )," = ",$s;