Fetching data with PDO - php

Question

I'm introducing myself to PDO and i'm trying to fetch data with it. I've done this before, but now i'm getting errors all the time. I've been through this for some hours and didn't find a mistake. If someone could help: my code is:
$tabelas_intervalos_afunda = ($con -> query('CREATE TABLE IF NOT EXISTS afunda_$a
SELECT (L1_forma_tensao_max + L1_forma_tensao_min)/2 as L1_forma_tensao, (L2_forma_tensao_max + L2_forma_tensao_min)/2 as L2_forma_tensao, (L3_forma_tensao_max + L3_forma_tensao_min)/2 as L3_forma_tensao
FROM afundamento
WHERE id > $prevNum AND id < $a');
while($row=$tabelas_intervalos_afunda->fetch(PDO::FETCH_ASSOC))
{
$array_forma_onda_fase1_afund[] = $row['L1_forma_tensao'];
$array_forma_onda_fase2_afund[] = $row['L2_forma_tensao'];
$array_forma_onda_fase3_afund[] = $row['L3_forma_tensao'];
}
My problem is that when i var_dump($array_forma_onda_fase1_afund), it returns me "Undefined variable $array_forma_onda_fase1_afund
NULL
Some additional info: $a is changed always when the loop condition is satisfied. The table afunda_$a is being created as expected, table afundamento exists normally.
Would appreciate any help/suggestions.

Answer 1

You are running a create table statement, which returns no rows. Then running fetch on the result, which doesn't enter the while statement. Hence $array_forma_onda_fase1_afund is never defined.
If you want the records you are inserting, you can select them out of the new table or run the original select query. E.g.:
$con->query('CREATE TABLE IF NOT EXISTS afunda_$a
SELECT (L1_forma_tensao_max + L1_forma_tensao_min)/2 as L1_forma_tensao, (L2_forma_tensao_max + L2_forma_tensao_min)/2 as L2_forma_tensao, (L3_forma_tensao_max + L3_forma_tensao_min)/2 as L3_forma_tensao
FROM afundamento
WHERE id > $prevNum AND id < $a');
$tabelas_intervalos_afunda = $con->query("SELECT * FROM afunda_$a");
while($row=$tabelas_intervalos_afunda->fetch(PDO::FETCH_ASSOC))
{
$array_forma_onda_fase1_afund[] = $row['L1_forma_tensao'];
$array_forma_onda_fase2_afund[] = $row['L2_forma_tensao'];
$array_forma_onda_fase3_afund[] = $row['L3_forma_tensao'];
}