[Resolved] simple thing; how does PHP interpret this?

[citric]

I'm self-teaching from a book and don't have other programming experience; so I know this is simple, but I think I'm missing something fundamental. It'd be a big help if someone could explain how PHP is interpreting this line of code:

if ($_REQUEST['file_name'] !== ('test1' || 'test2' ) )
{
  echo "bad file name<br>";
  exit;
}

echo "good file name<br>";

I've tried '!=' and '!=='. '!=' makes any file_name false ('good file name'). '!==' makes any file_name true ('bad file name'). If I take out the || comparison and only use one variable, it works. I think PHP is interpreting this statement as some kind of boolean true/false comparison; but I can't wrap my head around it, or understand why the || comparison breaks it.

(I'm making my own exercises to help remember basic commands, and this code is a very blunt attempt to restrict filenames so nobody can flood my account using my Astounding File Machine (http://plastickitten.org/testing3.html))

[TheDefender]

What happens if you break up the OR ( || ) like this:

if ($_REQUEST['file_name'] != 'test1' || $_REQUEST['file_name'] != 'test2' ) { 
  echo "bad file name<br>"; 
  exit; 
} else  {

echo "good file name<br>";
}

[citric]

Thanks for the reply.

Oddly, that doesn't work either. Actually, it breaks test1 as a good file name too.

if ( ($_REQUEST['file_name'] !== 'test1') || ($_REQUEST['file_name'] !== 'test2') )
{
  echo "bad file name<br>";
  echo $_REQUEST['file_name']; 
  exit;
}

echo "good file name<br>";

I'm testing it at http://plastickitten.org/testing4.php using variables through the address bar ('?file_name=test2'). Using that code, both test1 and test2 are "bad file names".

This is why I think I'm missing something fundamental. I don't understand how PHP is reading that line.

[laserlight]

Let's consider the expression:
$_REQUEST['file_name'] !== ('test1' || 'test2')

We start by evaluating ('test1' || 'test2')
Consider what the PHP Manual states in:
http://www.php.net/manual/en/language.types.boolean.php
You will find that 'test1' evaluates to true, and 'test2' evaluates to true.
(true || true) evaluates to true.

As such, we can simplify the expression to:
$_REQUEST['file_name'] !== true

Now, != means not equal to, while !== means not equal to or of different type:
http://www.php.net/manual/en/language.operators.comparison.php
Now, $_REQUEST['file_name'] is a string, and it is being compared to a boolean value.
As such, it is of a different type, so automatically the expression evaluates to true.
As such, you get the "bad file name" message.

Now, $REQUEST['file_name'] evaluates to true.
So for this:
$REQUEST['file_name'] != ('test1' || 'test2')
We simplify to:
$_REQUEST['file_name'] != true
And then to:
true != true
which of course is false, so "bad file name" does not get printed.

However, if $_REQUEST['file_name'] contains a false value, e.g. 0, we get
false != true, which is true, so "bad file name" gets printed.

[TheDefender]

Recheck what I posted, because your code doesn't match mine... Did you code like I gave you and try that? If so, how'd that work out?

[laserlight]

A more suitable example would be this:

<?php
//check that file_name is sent
if (isset($_REQUEST['file_name']))
{
	//check that file_name is valid
	if ($_REQUEST['file_name'] == 'test1' || $_REQUEST['file_name'] == 'test2')
	{
		echo "Good file name";
	}
	else
	{
		echo "Bad file name";
	}
}
else
{
	echo "No file name specified";
}
?>

Now, in the case that you're comparing with quite a few valid file names, then a switch/case selection structure would be more appropriate:

<?php
//check that file_name is sent
if (isset($_REQUEST['file_name']))
{
	//check that file_name is valid
	switch ($_REQUEST['file_name'])
	{
	case 'test1':
	case 'test2':
	case 'test3':
		echo "Good file name";
		break;
	default:
		echo "Bad file name";
	}
}
else
{
	echo "No file name specified";
}
?>

Other methods include placing the valid file names into an array, then searching the array with the given file name.
If no search results are returned, then the file name given must be invalid.

[citric]

Originally posted by TheDefender
Recheck what I posted, because your code doesn't match mine... Did you code like I gave you and try that? If so, how'd that work out?

True, I changed your code. Trying it exactly (this is a copy-paste):

if ($_REQUEST['file_name'] != 'test1' || $_REQUEST['file_name'] != 'test2' )
{
  echo "bad file name<br>";
  echo $_REQUEST['file_name']; 
  exit;
}

echo "good file name<br>";

It still doesn't work, though. But thanks anyhow.

[citric]

Laserlight: Thanks very much! I thought it was a boolean resolution thing; I still don't know why using || changed my results sometimes, but that can probably be chalked up to general confusion on my part.

I appreciate the long explanation, and also the solutions. Thanks -- changing the thread to resolved.

[laserlight]

Read the PHP Manual, I posted the link.

== is akin to !=
=== is akin to !===

For == and !=, the type isnt taken into consideration, as opposed to === and !==

But TheDefender's solution has a logic problem:
If the file name provided does not match the first test case, it will never match the rest due to lazy evaluation.
In other words, for the file 'test2', the "bas file name" message will still be printed.
Swap to $_REQUEST['file_name'] != 'test2' first and it will not be printed.

[citric]

Ohhh. Right.

Sorry, that last one was kind of a dumb question. Thanks a lot!

I'll go take a break and continue on with a clear head.

Thanks again. (I deleted the other thread)