مشکل در intellijidea

 <?php
if(isset($_REQUEST['action'])){
 $action=$_REQUEST['action'];
}else{
 echo "Invalid data";
 exit;
}
if($action=="read"){
 readData();
}


function connectionToData(){
 $connection=mysqli_connect("localhost", "root", "", "market_db");
 if (mysqli_connect_errno()) {
 echo "Failed to connect to MySQL: " . mysqli_connect_error();
 }
return $connection;
}

function readData(){
 $connection=connectionToData();//baargharari ertebat conection
 $result = mysqli_query($connection, "SELECT * FROM app WHERE " .$_REQUEST['filter'] ."ORDER BY".$_REQUEST['sort'] );

 $output = array();
 while($row = mysqli_fetch_array($result)) {
 $record = array();
 $record['app_id'] = $row['app_id'];
 $record['app_name']=$row['app_name'];
 $record['app_price'] = $row['app_price'];
 $record['app_logoUrl'] = $row['app_logoUrl'];
 $record['app_ratingValue'] = $row['app_ratingValue'];
 $record['app_author'] = $row['app_author'];
 $record['app_votes'] = $row['app_votes'];
 $record['app_fileSize'] = $row['app_fileSize'];
 $record['app_versionName'] = $row['app_versionName'];
 $record['app_installs'] = $row['app_installs'];
 $record['app_dec'] = $row['app_dec'];
 $record['app_phoneNumber'] = $row['app_phoneNumber'];
 $record['app_email'] = $row['app_email'];

 $output[] = $record;
 }

 echo json_encode($output);
 mysqli_close($connection);

}

این کد نمیدونم مشکل از چی هست