[PHP] Registro Online

<?php

    $USUARIO = "root" ;
    $SENHA = "";
    $BD = "SAMP";
    $HOST = "localhost";

    mysql_connect ($HOST, $USUARIO, $SENHA ) or die (mysql_error());
    mysql_select_db ($BD) or die (mysql_error());
?>


<?php
if(isset($_POST['acao'])) {

    $user      = $_POST['nick'];
    $email     = $_POST['email'];
    $password  = $_POST['senha'];
    $bool      = false ;

    if ( $user == "" or substr_count ( $user, "_" ) != 1 )
    {
        echo "<script>alert ('Campo Nome, incorreto. Use Nick_Sobrenick'); location.href='index.php'</script>";
        $bool = true ;
    }
    if ( $email == "" or substr_count ($email, "@")  != 1 or substr_count ($email, ".")  > 3 )
    {
        echo "<script>alert ('Campo E-mail, incorreto. Exemplo: example@email.com'); location.href='index.php'</script>";
        $bool = true ;
    }
    if ( $password == "" )
    {
        echo "<script>alert ('Preencha o campo Senha'); location.href='index.php'</script>";
        $bool = true ;
    }
    if ( strlen ( $password) > 21 or strlen ( $password ) < 5 )
    {
        echo "<script>alert ('Senha deve ser entre 5 e 21'); location.href='index.php'</script>";
        $bool = true ;
    }
    if ( strlen ( $user) > 21 or strlen ( $user ) < 5 )
    {
        echo "<script>alert ('Usuário deve ser entre 5 e 21'); location.href='index.php'</script>";
        $bool = true ;
    }
    if ( $bool == false )
    {
        $verificaUsuario = mysql_query("SELECT * FROM usuarios WHERE Nick = '$user' AND Email = '$email'");

        if(mysql_num_rows($verificaUsuario) > 0)   {

           $bool = true ;
            echo "<script>alert ('Nome de usuário ou e-mail já está cadastrado');</script>";
        }
        else   {

            $bool = true ;
            mysql_query("INSERT INTO `usuarios` ( `Nick`, `Password`, `Email`) VALUES ( '$user', '$password', '$email')") or die (mysql_error());

            echo "<script>alert ('Nick registrado com Sucesso: Nick: $user Senha: $password E-mail: $email'); location.href='index.php'</script>";
        }
    }
}
?>