首先，您应该在HTML代码中为Select标记设置一个名称:

First of all you should set a name for Select tag in HTML code:

echo '<select name="job">';

然后您可以使用$_POST变量获取所选选项:

Then you can get selected option using $_POST variable:

$ins = "INSERT INTO $db_table (job_name) VALUES ('" . mysql_escape_string($_POST['job']) . "')";