How do I solve the "Undefined variable" error on php [duplicate] - php

Question

This question already has answers here:
"Notice: Undefined variable", "Notice: Undefined index", "Warning: Undefined array key", and "Notice: Undefined offset" using PHP
(29 answers)
Closed 6 years ago.
login.php
<?php
function login()
{
$con = mysqli_connect("localhost", "XXX", "XXX") or die('Could not connect to server');
mysqli_select_db('$con', "store") or die('Could not connect to database');
}
?>
validate.php
Line 10 - ERROR - Notice: Undefined variable: con in C:\wamp\www\store\admin\validate.php
Line 10 - Warning: mysqli_query() expects parameter 1 to be mysqli, null given in C:\wamp\www\store\admin\validate.php
Line 12 - Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, null given in C:\wamp\www\store\admin\validate.php
<?php
session_start();
include ("../mylibrary/login.php");
login();
$userid = $_POST['userid'];
$password = $_POST['password'];
$query = "SELECT userid, name from admins where userid = '$userid' and password = PASSWORD('$password')";
$result = mysqli_query($con, $query); (**Line 10)
if (mysqli_num_rows($result) == 0)(**Line 12)
{
echo "<h5>Sorry, your account was not validated.</h5><br>\n";
echo "Try again<br>\n";
} else
{
$_SESSION['store_admin'] = $userid;
header("Location: admin.php");
}
?>
I tried to figure out something wrong. Let me know thanks.

Answer 1

See this
mysqli_select_db('$con'
^ ^
Variable don't get parsed in single quotes.
Either remove them or use double " quotes.
Note: Make sure that all your POST arrays contain values and that the form you're using is indeed using a POST method and elements hold their respective name attributes.
You're also open to an SQL injection here.
Use a prepared statement.
References:
https://en.wikipedia.org/wiki/SQL_injection
https://en.wikipedia.org/wiki/Prepared_statement
Also consider using password_hash() to store your passwords:
http://php.net/manual/en/function.password-hash.php (Read the entire manual).

Answer 2

You are mixing up local and global variables.
$con is not available outside the login function.
If you add global $con; as the first line of the login function, its available outside of the function as well.
Have a look at the php manual: http://php.net/manual/en/language.variables.scope.php
PS: While its not the source of the problem, read Fred's answer too.