为什么我会收到“致命错误：函数名称必须是第8行的字符串”[关闭]

I have try many times but I'm not sure why I get error saying that "Fatal error: Function name must be a string in view_registration2.php on line 8". Can someone tell me whats wrong? Did I use wrong syntax somewhere? Thank you :)

   //This is my other php file 
   //<FORM name="form1" method ="POST" action="view_registration2.php" >
   //<select name="semester">
   //   <option value="" selected> -- choose which semester you want to list --
   //   <option value="06072">06072
   //   <option value="06071">06071
   //</select>
   //<INPUT type="submit" name="button1" value="Submit">

$sem = $_POST("semester");  //line 8

echo $sem;
require_once("dbconn.php");

$query1 = mysql_query("select * from  student");
if(!$query1) die("SQL query error encountered : " . mysql_error() );
$matrik1= array(); $nama1 = array();
while($record = mysql_fetch_array ($query1))
{
    $matrik1[]=$record['matric'];
    $nama1[]=$record['name'];
}

The $_POST superglobal is an Array, not a function. You access its indexes with square brackets [] not parenthesis ().

$sem = $_POST["semester"];