将PHP数组传递给jQuery会返回空数组或通知

I'm trying to read some information from a MySQL database using php and then pass the results to jquery and log them in the console.

To read from the database I use:

public function getQuestions(){
    $query = 'SELECT * FROM Question';
    $statement = $this->db->prepare($query);
    $statement->execute();
    $questions = [];
    while($row = $statement->fetch()){
        $questions[] = array(
            'QuestionID' => $row['QuestionID'],
            'Content' => $row['QuestionContent']
        );
    }
    return $questions;
}

where $this->db is a PDO object.

Then this method is called from

<?php

include_once 'classes/DatabaseAdapter.php';

$dba = new DatabaseAdapter();
echo $dba->getQuestions(); // Array to String conversion error.

And that is then called from jQuery with:

$.ajax({
    url: 'questions.php',
    method: 'post'
}).done(function(data){
    console.log('done');
    console.log(data);
});

The problem I'm having is when I encode the output as json in PHP with json_encode() I get an empty string in the console and decode it in jQuery with JSON.parse(). Then I try passing it without encoding and get a notice saying Array to String conversion in PHP line with echo.

Try this way. You can adapt the code to your getQuestions() function

Php code

//PDO connection

$host   = "";
$user   = "";
$dbname = "";
$pass   = "";

try {
  $DBH = new PDO("mysql:host=$host;dbname=$dbname;charset=utf8", $user, $pass,
                    array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8'"));
}

//PDO Database query and Json encode

$STH = $DBH->prepare("SELECT * FROM Question");
$STH->execute();
$result = $STH->fetchAll();
$data = array();
$data['items'] = $result;
echo json_encode($data);

Jquery

//$.getJSON is a shorthand Ajax function

$.getJSON("questions.php", function(data) {
console.log(data);
$.each(data.items, function(i, value){
console.log(value.thecolumnnameinmysqltable);
})
})

In the Php im creating an array for the purpose that you can add extra data and retrieve it .eg

$data = array();
$data['data'] = array("one", "two", "three", "four");
$data['items'] = $result;
echo json_encode($data);

and to retrieve the $data['data'] array in Jquery you do

    var one   = data.data[0];
    var two   = data.data[1];
    var three = data.data[2];
    var four  = data.data[3];

Getjson info -- http://api.jquery.com/jquery.getjson/

Try this.

echo json_encode($dba->getQuestions());
exit;