从js获取值到sql变量

Question

我需要一些帮助来弄清楚如何将值用于我用于sql语句的变量

我的js值来自

$('#journalName').on('click','.clickme',function(){
    alert($(this).attr('value'));

    var a=$(this).attr('value');
    $.post("../php/viewThisJournal.php",{val:a},function(data){});
    location.href="../subpages/viewThisJournalLogin.php" ;
});

我需要将值输入sql变量$ selectedJournal，我在我的php文件viewThisJournal.php中使用它当前持有值27

$selectedJournal='27';
$loggedinuser=$_SESSION["email"];
$sql="SELECT ChapterTbl.heading, ChapterTbl.bodyText FROM ChapterTbl,JournalTbl WHERE (ChapterTbl.parentJournal='$selectedJournal') AND (JournalTbl.owner='$loggedinuser')";

try {
    $st = $conn->prepare($sql); 
    $st->execute();
    $rows=$st->fetchAll();
} catch (PDOException $e) {
    echo "Server Error - try again!".$e->getMessage();
};

echo json_encode($rows);


?>

提前致谢