Przejście z php 5.2 do php7 - błędy...

$query=mysqli_query($sql="$con, DELETE FROM {$this->session_tbl} WHERE session_end<".time()); 

$db_host = '...';
$db_user = '...'; 
$db_pass = '...';
$db_name = '...';
$con = mysqli_connect($db_host, $db_user, $db_pass, $db_name) or die( 'BŁĄD POŁĄCZENIA z MySQL');

$sql="$con, DELETE FROM {$this->session_tbl} WHERE session_end<"

// zamiast tego zrób
$query=mysqli_query($con, "DELETE FROM {$this->session_tbl} WHERE session_end<".time()); 

$query=mysqli_query($sql="$con, DELETE FROM {$this->session_tbl} WHERE session_end<".time()); 

Ciekawe, przecież to nie miało prawa działać również na php 5

$result = mysqli_query($con, "SELECT * FROM forum_a WHERE topic='-' ORDER BY reply DESC LIMIT $limit");
$r = mysqli_fetch_object($result);

$query = "SELECT * FROM forum_a WHERE topic='-' AND cat <> 14 ORDER BY reply DESC LIMIT $limit";
$result = mysqli_query($con, $query);

$tresc = mysqli_result(mysqli_query($con, "SELECT text FROM forum_b WHERE id='$id'"), 0);
$idsss = @mysqli_result(mysqli_query($con, "SELECT id FROM forum_a WHERE topic='$id' ORDER BY date DESC"), 0);
$tresc = @mysqli_result(mysqli_query($con, "SELECT text FROM forum_b WHERE id='$idsss'"), 0);

if ($idsssIlosc==0) {
$tresc = mysqli_result(mysqli_query($con, "SELECT text FROM forum_b WHERE id='$id'"), 0);
}elseif($idsssIlosc>0) {

//$idsss = mysqli_result(mysqli_query($con, "SELECT id FROM forum_a WHERE topic='$id' ORDER BY date DESC"), 0);
$sql_idsss = "SELECT id FROM forum_a WHERE topic='$id' ORDER BY date DESC";
$idsss = mysqli_query($con, $sql_idsss);

//$tresc = mysqli_result(mysqli_query($con, "SELECT text FROM forum_b WHERE id='$idsss'"), 0);

$sql_tresc = "SELECT text FROM forum_b WHERE id='$idsss'"; // 471
$tresc = mysqli_query($con, $sql_tresc);
}

1' OR '2'

 $id = htmlspecialchars($id) 

SELECT id FROM forum_a WHERE topic='$id' ORDER BY date DESC;

//nowe połączenie pod zmienną pdo
$pdo = new PDO(
    'mysql:host=localhost;dbname=[TUTAJ DATABASE NAME]',
    'username',
    'password');

//pod zmienną $stmt przygotowujemy baze pod zapytanie
$stmt = $pdo->prepare('SELECT id FROM forum_a WHERE topic= :topic ORDER BY date DESC;');

//bindParam do zapytania wyżej, za :topic, wchodzi $id
$stmt->bindParam(':topic', $id);

//Wykonanie zapytania
$stmt->execute();

//FETCH - wiadomo co robi, jest także fetchAll()
$fetch = $stmt->fetch();

//zamykamy $stmt, połączenie z bazą dalej jest, można zrobic nowe $pdo->prepare (kolejne zapytanie)
$stmt->closeCursor();

//zamykamy całkowicie połączenie z bazą
$pdo = null;

var_dump($fetch);