If you send data using GET method within your url you should update your query in main.php getting:
$day = $_GET['year'];
$month = $_GET['month'];
$year = $_GET['day'];
$sql = "SELECT HoraIni,MinutoIni,HoraFim,MinutoFim,CdCurso,NmCurso,DgTpMarcacao FROM marcacaosalas Where Data="$year-$month-$day"";
Anyway i dont understand why you don’t easly send the whole data to the page simpli using a fomith a date input inside like:
<form method="get">
<input type="date" name="date" />
<input type="submit" value="submit" />
</form>
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solved How to run a query with the values of a windows open PHP