[hint] : SQLi3 Challenge By ABaDy

<?php
/* ABaDy SQLi Challenge; Level:M */
$sql = new mysqli("localhost", "conanCTF", "HelloWorld", "conanCTF");
$f = '@(([\'"]).*?[^\\\]\2)|((?:\#|--).*?$|/\*(?:[^/*]|/(?!\*)|\*(?!/)|(?R))*\*\/)\s*|(?<=;)\s+@ms';
$id = $sql->real_escape_string($_GET['id']);
$id = trim(preg_replace($f, '$1', $id));
$q = $sql->query("SELECT * FROM `flags` WHERE (id is not null) and (id={$id} and u_id=1)");
$flag = ($q->num_rows)?$q->fetch_assoc()['flag']:'not found';
if(!isset($_GET['id']) or empty($_GET['id'])):
    header('LOCATION: ./index.php?id=1');
    exit();
endif;
?>
<!DOCTYPE html>
<html>
<body style="background-color:#FFFFFF;">
<center><img src="Conan.png" alt=";)" height="23%" width="23%"><br><b><font size='7'>sh0w me wh4t y0u c4n d0, </font><font size='7' color='red'>gen!us</font><font size='7'> !</font></b>
<br><br><br>
<b><font size='6' color='red'><? echo $flag; ?></font></b>
</center>
</body>
</html>