Why can't I use $app in called function for Slim?

Easy way to fix this is to use getInstance();

function yourFunction(){   $app = \Slim\Slim::getInstance();}

You could always pass the $app object as an argument to your function.

$app = new \Slim\Slim();//GET CHAPTERS$app->get(    '/chapters',    function () use ($app) {        executeSql($app, 'SELECT * FROM chapters ORDER BY id');    });//GENERIC SQL EXECUTEfunction executeSql(\Slim\Slim $app, $sql) {        $app->contentType('application/json');    try {        $db = getConnection();        $stmt = $db->query($sql);          $results = $stmt->fetchAll(PDO::FETCH_OBJ);        $db = null;        echo json_encode($results);    } catch(PDOException $e) {        echo '{"error":{"text":'. $e->getMessage() .'}}';     }}

It is cleaner to keep database calls and app logic/presentation separate. I would return your "chapters" object immediately and handle anything else outside of the executeSql function.

$app = new \Slim\Slim();// GET CHAPTERS$app->get('/chapters', function () use ($app) {  $app->contentType('application/json');  $chapters = executeSql('SELECT * FROM chapters ORDER BY id');  if ($chapters) {    $app->response->setStatus(200);    echo json_encode($results);  } else {    $app->response->setStatus(400);    echo '{"error":{"text":"error getting chapters"}}';  }});// GENERIC SQL EXECUTEfunction executeSql($sql) {  try {    $db = getConnection();    $stmt = $db->query($sql);      $results = $stmt->fetchAll(PDO::FETCH_OBJ);    $db = null;    return $results;  } catch(PDOException $e) {    echo '{"error":{"text":'. $e->getMessage() .'}}';   }}