怎样叠加变量？

[zairwolf]

$step=1;
$color($step)="red";
echo($color1);

为什么没有显示？

[chinaworker]

$color."{$step}"="red";

[zairwolf]

Parse error: parse error, unexpected '=' in /usr/home/bbs.soulsky.net/public_html/test.php on line 3

[chinaworker]

<?

$step='1';
${"color".$step} = 'red';
echo $color1;

?>