php Parse error: syntax error, unexpected T_VARIABLE in 错误

Question

<?php
session_start();
$username=$_post[user];
$sql=mysql_query("select*from luntan where regname="$username"");
$result=mysql_fetch_array($sql);
if($result!=flase){
echo("alert(&#39;用户名已被注册！&#39;);history.go(-1);");
exit();
}
$password=$_post[password];
$password2=$_post[password2];
if($password!=$password2){
echo("alert(&#39;两次输入密码不同！&#39;);history.go(-1);");
exit();
else
$INS=mysql("Inset Into skyshadow (user,passwrod) Values ('$username','$password')");
echo "alert(&#39;用户注册成功!&#39;);";
echo "windows.location=&quot;H:\论坛\index.html&quot;;";
}
?>

自己第一次写的网站代码 不知道哪里错了