isset($_POST[]) not catching blank form fields

bunner_bob

Maybe this is how it's supposed to work, but I have a form with a checkbox, which is often left unchecked, so when it's submitted it sends the value '' (empty quotes) - I think that's what it sends anyway...

In my processing script, if I have this:
if (isset($_POST['checkbox'])) {
echo "foo";
}
which does nothing if the checkbox is unchecked.

However, this DOES:
if ($_POST['checkbox'] == '') {
echo "foo";
}

Shouldn't isset() catch an empty array element?

LordShryku

Not if it's present. [man]empty[/man] would though.

bodzan

isiset cannot find the variable checkbox because when it is not checked it doesn't exist!

However, when you check whether $_POST[checkbox] is '' it returns true because it is not set to anything...

Weedpacket

if 'checkbox' is an unchecked checkbox, then isset($POST['checkbox']) will evaluate to false, because $POST['checkbox'] is not set (unchecked checkboxes are not including in the returned form data).

($POST['checkbox']=='') will do two things: since $POST['checkbox'] does not exist, PHP will assume that it is empty - the value of an empty variable is equivalent to null, false, 0, or an empty string. An empty string is equal to an empty string, so the test succeeds.

Also, if you code with error reporting set to E_ALL (which you should be doing during development at the very least), the second example will throw a notice-level message about your using an uninitialised array index - a hint that your code needs improvement.

bunner_bob

Yes, I have been working with E_ALL on (and wishing I'd done it sooner, but it only took one long day to clear everything up).

Question - you say unchecked checkboxes are not POSTed with the form. What about other fields - blank text fields or textareas?

Another question - when testing for $POST['submit'], is it enough to say

if (isset($_POST['submit'])) { stuff }

or should I be saying

if (isset($_POST['submit']) && $_POST['submit']) { stuff }

i.e. checking for both the existence of 'submit' AND a value in it?

I know this double-checking is necessary in certain cases (maybe a lot of cases) but nice to skip it when possible.

bodzan

The thing about checkboxes is that they do not exist unless they are checked. They are the only form element that has that kind of behaviour (and radio buttons but if you say that one of them is checked and they all have the same name you can get around this).

Text boxes option elements and other buy default exist even if nothing is entered in them.

The first example would work fine except you have another submit variable somewhere in the script.

Somebody please correct me if I figured these thing wrong.

Weedpacket

Originally posted by bunner bob
Question - you say unchecked checkboxes are not POSTed with the form. What about other fields - blank text fields or textareas?

Here's what the HTML specification has to say on the subject of form submission (starting from section 2 on "successful controls"):
http://www.w3.org/TR/html401/interact/forms.html#h-17.13.2
But it's a good idea to test for existence anyway - someone could try sending you something they've mangled in an attempt to make your script choke.

Another question - when testing for $POST['submit'], is it enough to say
if (isset($_POST['submit'])) { stuff }
or should I be saying
if (isset($_POST['submit']) && $_POST['submit']) { stuff }
[/b]

Depends on what sort of form field 'submit' is; but your latter code is equivalent to if(!empty($_POST['submit'])) so you can use that. (Well, that's a little bit of a lie; if the field contains '0', that qualifies as "empty" as well....)