PHP中if($_POST['submit'])为什么提交不了呢？

<?php
require("head.php");
if($_POST['submit']) {
$t = "insert into liuyan (id,yonghu,neirong,shijian) values ('','$_post[yh]','$_post[nr]',now())";
mysql_query($t);
echo "发表成功！";
}
?>

<form method="post" action="sub.php">
<p>
<input type="text" name="yh" />
</p>
<p>
<textarea name="nr"></textarea>
</p>
<p>
<input type="submit" name="Submit" value="提交" />
</p>
</form>
<?php
require ("foot.php");
?>

下面是head.php

<?php
$sqlhost = "localhost";
$user = "root";
$password = "";
$dbname = "blog";
$blogname = "唱一半的歌";
$sql = @mysql_connect($sqlhost,$user,$password) or die("数据库连接出错。");
mysql_select_db($dbname,$sql);
mysql_query("set names gb2312")
?>

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