$sql = "SELECT 'id', 'nome', 'nivel' FROM 'usuarios' WHERE ('usuario' = '".$usuario ."') AND ('senha' = '". sha1($senha) ."') AND ('ativo' = 1) LIMIT 1";
$result = mysqli_query($conn, $sql);
if (mysqli_num_rows($result) != 1) {
echo "Login inválido!"; exit;
} else {
//
}
Error:
Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in
The problem is that your query has an error, so mysqli_query returns false and passing this value to mysqli_num_rows causes error
Before the if prints on the screen the command ( echo $sql; ) and try to execute the same in phpMyAdmin, MySQL Workbench or something, these software will show more exactly where the error is
To know how many rows have come, you can try to do this:
$result = mysqli_query($conn, $sql);
$arrDados = mysqli_fetch_assoc($result);
$nLinhas = count($arrDados);
if ($nLinhas == 1) {
echo "Login Ok!";
// Continua sua programação.
} else {
echo "Login inválido!"; exit;
}