<?php
    $id = $_GET['id'];
    $sql = mysqli_query($con, "SELECT * FROM pages WHERE id='$id'");
?>

$id = (int) $_GET['id'];
$sql = mysqli_query($con, "SELECT * FROM pages WHERE id=$id");

$id = (int) $_GET['id'];
$stmt = mysqli_prepare($con, "SELECT * FROM pages WHERE id=?");
mysqli_stmt_bind_param($stmt, "i", $id);

filter_var($id,FILTER_VALIDATE_INT);