想用PHP实现一个点击改变颜色的网页，编辑器提示select有警告，运行之后只有html的代码，JavaScript的代码却没有在控制台显示出来，这是怎么回事？

<div id="shi">
竹杖芒鞋轻胜马<br>
</div>
<form action="$_POST.php" method="POST">
<select name="color" >
<option value="0">请选择颜色</option>
<option value="1">红色</option>
<option value="2">绿色</option>
<option value="3">蓝色</option>
</select>
<input type="submit" name='button' value="提交">
</form>
<?php
if(isset($_POST['button'])) {
$color = $_POST['color'];
switch ($color) {
case 1:
$color = '#ff0000';
break;
case 2:
$color = '#009900';
break;
case 3:
$color = '#0000ff';
break;
default:
$color = '#000000';
}
?>
<script type="text/javascript">
shi.style.color='<?php echo "$color"; ?>';
</script>

<?php
}
?>