<?php

$dbh = new PDO("mysql:host=$host;dbname=$dbname;charset=utf8", '$user', '$pass'); 
$stmt = $dbh->prepare('SELECT theme,question,reply1,reply2,reply3,reply4,correctly FROM test_questions WHERE theme=? ORDER BY RAND() LIMIT 0,15 ');

$results= array();
$questions = array();

foreach (explode(',', $_REQUEST['test_set']) as $ind => $thm) {	
	$results[$thm] = 0;	
	$stmt->execute([$thm,]);
	$questions = array_merge($questions, $stmt->fetchAll());
}

echo(json_encode(Array(
		"results" => 		$results,	
		"questions" => 		$questions,
		"num_questions" => 	count($questions),
		"is_end_test" => 		1,
	)));

?>

$stmt = $dbh->prepare(
	'SELECT test_questions.* FROM (
		SELECT id_question
		FROM gostest_questions
		WHERE theme=?
		ORDER BY RAND()
		LIMIT 15
	) AS ids JOIN test_questionsON test_questions.id_question = ids.id_question'
);