PHP error: "Trying to get property of non-object" - php

Question

I know this question has been asked alot of times earlier, but none of the other answers worked for me. I'm having trouble on this line:
$row = $conn->query("SELECT * FROM urls WHERE id = '$id'");
I followed a tutorial so I don't know if there is any other information that I should provide
EDIT:
heres the whole text document:
<?php
function idExists($id){
include $_SERVER['DOCUMENT_ROOT'] . '/short/includes/init.php';
$row = $conn->query("SELECT * FROM urls WHERE id = '.$id'");
if($row -> num_rows > 0){
return true;
} else {
return false;
}
}
function urlHasBeenShortened($url){
include $_SERVER['DOCUMENT_ROOT'] . '/short/includes/init.php';
$row = $conn->query("SELECT * FROM urls WHERE link_to_page = '$url'");
if($row->num_rows > 0){
return true;
} else {
return false;
}
}
function getURLID($url){
include $_SERVER['DOCUMENT_ROOT'] . '/short/includes/init.php';
$row = $conn->query("SELECT id FROM urls WHERE link_to_page = '$url'");
return $row->fetch_assoc()['id'];
}
function insertID($id, $url){
include $_SERVER['DOCUMENT_ROOT'] . '/short/includes/init.php';
$conn->query("INSERT INTO urls (id, link_to_page) VALUES ('$id', '$url')");
if(strlen($conn->error) == 0){
return true;
}
}
function getUrlLocation($id){
include $_SERVER['DOCUMENT_ROOT'] . '/short/includes/init.php';
$row = $conn->query("SELECT link_to_page FROM urls WHERE id = '$id'");
return $row->fetch_assoc()['link_to_page'];
}
?>
Init code
<?php
$servername = "localhost";
$username = "root";
$password = "";
// Create connection
$conn = new mysqli($servername, $username, $password);
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
echo "Connected successfully";
?>
errors on lines: 7, 18

Answer 1

you did forget the database name :
$conn = new mysqli($servername, $username, $password);
// Create connection like this :
$conn = new mysqli($servername, $username, $password, $dbname);
change
$row = $conn->query("SELECT * FROM urls WHERE id = '$id'");
to
$row = $conn->query("SELECT * FROM urls WHERE id = ".$id);
also change :
if($row -> num_rows > 0)
to
if($row->num_rows > 0)