Problem when checking if user already exists before registering

Question

I’m starting now, and I have a question. I had done to register a category, until then ok, but I realized that I was registering even with an existing category, then I decided to add a function to check the database if what was typed already has there, but I’m not getting.

<?php
session_start();
$adicionar_categoria = $_POST['categoria'];
include_once("../../config/conexao.php");

//Verifica se a categoria já existe.
    $vcategoria = mysql_query("SELECT * FROM categorias(nomeCategoria) LIMIT 1");

//Se o retorno for maior que 0, diz que já existe uma categoria com o mesmo nome.
if(mysql_num_rows($vcategoria) > 0){
    $_SESSION['categoriaErro'] = "Categoria existente.";
    header("Location: ../adicionar_categoria.php");
}else{
$resultCategoria = mysql_query("INSERT INTO categorias(nomeCategoria) VALUES ('$adicionar_categoria')");
}
//echo "Categoria: ".$resultCategoria['nomeCategoria'];

if(empty($resultCategoria)){
    $_SESSION['categoriaErro'] = "Por favor, adicione uma categoria válida.";
    header("Location: ../adicionar_categoria.php");
}else{
    $_SESSION['categoriaSucesso'] = "Categoria adicionada com sucesso.";
    header("Location: ../adicionar_categoria.php");
}
?>

Answer 1

Your search query is wrong, it should be so:

SELECT * FROM categorias WHERE nomeCategoria = '$adicionar_categoria' LIMIT 1

HOWEVER: your code has serious compatibility and safety problems. I strongly suggest consulting this other question: Why should we not use mysql type functions_*?