Best Fix Found for Random Deletions in "Classes" Database?

Hi,

Random and completely unasked for delections have been occurring in my "Classes" database.
I would like to know if I have correctly analysed and handled the situation.

I first filter on SecurityAssist_ID to create a "View" page of all classes per each Presenter.

I then drill down to details per each specified class on an "Edit" Page using:

edit_class.php?ClassID=<?php echo($rsGetClassInfo->getColumnVal("ClassID")); ?>

This (of course) gives me a:
$_GET['ClassID'],
to go with that
'ClassID=xxxx'.

But if a Presenter merely looks at the page then backs out (doing nothing), the record was deleted.
I decided that this was probably because nothing or null was somehow being used as an unauthorized value for an update.

I then noticed MySQL's coding for the Update begins with:
if (true) {
blah blah blah;
}

I therefore amended this to:

<?php
if (isset($_POST["ClassLength"])) {
blah blah blah
}
(with there being a different such page for every class attribute, that Post being linked to the ClassID from the GET).

It may be a coincidence, but there has not been a repeat since. So unless I am mistaken, amending that "if" clause cured the problem.

Does this mean I must in the future amend all such "if (true)" MySQLi update queries that involve a ?ClassID = xxxx appended to the URL?

This is surely such a common scenario that it seems strange it should be so?
But if it is so, then I am sure my above code fragment is primitive in the extreme.
Could you please suggest what more auspicious expression I should be using?

Thank you,
KAB