Check PDO query return to delete or insert records

I am making a select for deletar or inserir records in my bd according to the amount of records returned, but the tests I have done are not correct, I do not know exactly how to count the records in a return pdo . I've already used count and it did not work, I think I'm making a lot of mistakes when trying.

I'll post the commented code to help with understanding, the code is this:

// VERIFICANDO DIREITOS 
$sqlInterface = "    
    SELECT 
      gasUsuarioServico.IdUsuario,
      gasUsuarioServico.IdServico,
      gasServico.IdInterface
    FROM
      gasUsuarioServico
      INNER JOIN gasServico ON (gasUsuarioServico.IdServico = gasServico.IdServico)
    WHERE
      (gasUsuarioServico.IdUsuario = ?) AND 
      (gasServico.IdInterface = ?)";
$stm = $pdo->prepare($sqlInterface);
// DEFINE O TIPO DA VARIÁVEL PDO::PARAM_INT OU STR
$stm->bindValue(1, $IdUsuario, PDO::PARAM_INT);
$stm->bindValue(2, $IdInterface, PDO::PARAM_INT);
$stm->execute();    
$registros = $stm->fetchAll(PDO::FETCH_ASSOC); 
// TENTATIVA DE CONTAGEM DE REGISTROS RETORNADOS
$conSqlDel = count($registros);    

if( $conSqlDel == 0 ){

    $sqlDel = "DELETE FROM gasUsuarioInterface WHERE gasUsuarioInterface.IdUsuario = ? AND gasUsuarioInterface.IdInterface = ?";
    $stm = $pdo->prepare($sqlDel);        
    $stm->bindValue(1, $IdUsuario, PDO::PARAM_INT);
    $stm->bindValue(2, $IdInterface, PDO::PARAM_INT);                    
    $retorno = $stm->execute();  

} else {

    $sqlInterfaceReg = "    
        SELECT *
        FROM
            gasUsuarioInterface
        WHERE
            (gasUsuarioInterface.IdUsuario = ?) AND 
            (gasUsuarioInterface.IdInterface = ?)";
    $stm = $pdo->prepare($sqlInterfaceReg);
    // DEFINE O TIPO DA VARIÁVEL PDO::PARAM_INT OU STR
    $stm->bindValue(1, $IdUsuario, PDO::PARAM_INT);
    $stm->bindValue(2, $IdInterface, PDO::PARAM_INT);
    $stm->execute();        
    $registros = $stm->fetchAll(PDO::FETCH_ASSOC); 
    $countInsert = count($registros);

    if ($countInsert == 0) {            
        $arrayDados = array('IdUsuario' => $IdUsuario, 'IdServico' => $IdInterface );              
        $retorno = $crud->insert($arrayDados);   
    }    
} 

In the test I made the variable $ conSqlDel return 2 records, but in my bd the value is zero, that is, no records returned.

I'm sending a JSON return, like this:

    // MENSAGEM AO USUÁRIO
if ($retorno):
    $retorno = array('codigo' => 1, 'mensagem' =>  print_r($conSqlDel) );
    echo json_encode($retorno);
    exit();
else:
    $retorno = array('codigo' => '0', 'mensagem' => ' Registro não foi atualizado' );
    echo json_encode($retorno);
    exit();
endif;

I do not know how to check the contents of the variables, since a simple echo or print_r does not work.