Convert PHP MySQL to PHP PDO - php

Question

I am trying to convert this PHP MySQL code from:
require_once("dbcontroller.php");
$db_handle = new DBController();
if(!empty($_POST["primary_cat"])) {
$query ="SELECT DISTINCT `secondary_cat` FROM student5 WHERE primary_cat = '" . $_POST["primary_cat"] . "'";
$results = $db_handle->runQuery($query);
?>
<option value="">Select State</option>
<?php
foreach($results as $state) {
?>
<option value="<?php echo $state["secondary_cat"]; ?>"><?php echo $state["secondary_cat"]; ?></option>
<?php
}
}
to PHP PDO as follows. I'm not sure what I am doing wrong. This is my code below.
$table = 'student5';
include('config.php');
if(!empty($_POST['primary_cat'])) {
$sqlQuerySecondaryCat = $dbh->query("SELECT DISTINCT secondary_cat FROM $table WHERE primary_cat = :primary_cat");
$sqlQuerySecondaryCat->execute(array(':primary_cat' => $_POST['primary_cat']));
?>
<option value="">Select State</option>
<?php
foreach($sqlQuerySecondaryCat as $secondaryCatRow) {
?>
<option value="<?php echo $secondaryCatRow["secondary_cat"]; ?>"><?php echo $secondaryCatRow["secondary_cat"]; ?></option>
<?php
}
}
I am getting the following error in my error_log PHP Warning: Invalid argument supplied for foreach()

Answer 1

First of all you need to use prepare, not query.
$sqlQuerySecondaryCat->prepare("SELECT .......
Secondly, you haven't done anything with the data stored in the object $sqlQuerySecondaryCat - you need to use fetch or fetchAll for the data.
$data = $sqlQuerySecondaryCat->fetchAll(PDO::FETCH_ASSOC);
foreach($data as $secondaryCatRow){
.... do something
Full code:
$table = 'student5';
include('config.php');
if(!empty($_POST['primary_cat'])) {
$sqlQuerySecondaryCat = $dbh->prepare("SELECT DISTINCT secondary_cat FROM $table WHERE primary_cat = :primary_cat");
$sqlQuerySecondaryCat->execute(array(':primary_cat' => $_POST['primary_cat']));
$rows = $sqlQuerySecondaryCat->fetchAll(PDO::FETCH_ASSOC); ?>
<option value="">Select State</option>
<?php foreach($rows as $secondaryCatRow) { ?>
<option value="<?php echo $secondaryCatRow["secondary_cat"]; ?>"><?php echo $secondaryCatRow["secondary_cat"]; ?></option>
<?php }
}