无法解决MySQL语法中的错误 - 工作但显示错误消息

After many hours of trying to understand what is wrong with this simple query, is there anything that I'm missing? It posts correctly, and everything works fine (e.g. it updates the DB with the variables correctly) but displays the following error every time upon submission:

Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '1' at line 1

    $eventIDResult = ((int)$_POST["eventIDResult"]);
$eventNameResult = ($_POST["eventNameResult"]);
$eventTextResult = ($_POST["eventTextResult"]);
$eventDateResult = ($_POST["eventDateResult"]);
$eventAgendaResult = ($_POST["eventAgendaResult"]);
$sql = mysql_query("UPDATE events SET eventName='$eventNameResult', eventText='$eventTextResult', eventDate='$eventDateResult', eventAgenda='$eventAgendaResult' WHERE eventID='$eventIDResult'");
if (!mysql_query($sql, $con))
{
die('Error: ' . mysql_error());
}
else
{
header('Location: ../add-remove-event.php');
exit;
}
mysql_close($con);

You're calling mysql_query on the result of the previous mysql_query, which probably returns the number of rows affected (in this case, 1):

$sql = mysql_query(...)
if (!mysql_query(...))

What you probably meant to do was just put the query in a string:

$sql = "...";
if (!mysql_query($sql, $con))
...

Moving the comment as an answer so viewers in the future do know what was the answer.

Make sure all the fields are varchar. ¿Are none of them INT or similar? ¿Is the eventID of INT type (and therefore, does not require single quotes)?