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In this movie we're going to talk about problems that can occur in your validation logic. Now these are problems that happen with any if statement, but they're critical to get right when we're working with validations. Normally in your code, if your if statement's not right, well, then you'll see it right there on the page and know, while you're developing, that you've done something wrong. But if our validations fail us, then we lose control over the data that's coming into our application. And we may not even know about it because we're not there at the time that it happens. So let's work to understand some of the common problems. The last movie we wrote a validation, that would check to make sure the @ symbol, was inside a string that was being submitted by a form.
And the string POS function is what we were using. That function either returns the position where the item is found, or it returns false if it's not found at all. If we use the simple comparison operator equals equals then we get false positives. We need to use the triple equals to say that it's exact. And the reason why is because if the @ symbol is in the first position, then string position will return zero to us. because that's the position the string starting at zero so it'll return zero and believe it or not zero is equal to false as far as PHP is concerned. Let's take a look.
Let's create ourselves a new page. I'm going to create basic.html, and I'm going to do a Save As, and let's call this false_positives.php. Now these aren't really truly false positives, because this is just the way PHP works. It's not like PHP is getting it incorrect, PHP is doing it correctly, but they might seem like false positives to us if we don't understand what it's doing. So, let me just start by pasting in a basic function here, and I'm going to call this function is equal. It's going to take two values, and it's going to output those values to us followed by a colon, using the double equals operator, and then it's going to compare them.
And if they are equal, then it's going to output true, and if not equal, it's going to output false. So it's going to return the output to us, and then, I'm just going to paste a whole bunch of examples here. Not going to sit and type them all out. You can take time to review them one by one. But I'm going to be comparing various things, like zero vs false. The number four vs true. Zero number vs the string and zero. Zero vs an empty string. Zero vs a string with the letter A in it. The number one vs the number 01, and so on.
Let's try this all out and see what we get. Go into Firefox and lets load up false positives dot php. Look at that. Every single one of them returned true. Every single one said it was true. Now don't worry the display here is a little off because when it goes to convert false to a string to display it, it comes back with no empty string so don't let that throw you. The key part is notice that every single one of these lines returns true. All of these things are considered equivalent in PHP. Now why is that? It's because when PHP does a comparison it does type juggling for you. Remember we talked about type juggling.
PHP does type juggling here in order to compare two things that have a different type. In every single one of these cases, I've got something that is of a different type, with one exception which is here. I've got something that are two strings, right? So be careful about that. Here, I've got things that are both strings but it compares them and says that they are equal because there's only spaces at the beginning or the plus sign at the beginning. And it sees those as being numbers. It's an odd quirk in PHP ,I just want you to see. So all of these things are considered equal. There are some rules for how PHP goes about doing these comparisons.
When PHP does type juggling during comparisons. String vs null. It converts null to be an empty string and then compares them to see if they are the same. So empty string versus null is going to be equal. The letters A, B, C versus null is not going to be equal. If we have a Boolean versus any other type at all, doesn't matter what it is, any other type it's going to convert the other one to be a Boolean. And there are rules about how it does that, but it's going to try and figure out how to convert. A number or a string to be in a boolean, either true or false. And then number versus others, the last one, if it's a number versus something else besides boolean, it's going to convert that other thing to being a number.
So if that other thing is a string, it'll use its conversion to convert a string to a number. We saw a little bit of how that worked earlier. If it's null, well, it's going to try and convert that to be a number. What is null? Well, that's converted to 0. So these are the basic rules that it's going to use to follow when it's doing the switching. So how can you get around these false positives? Let's just take all of this code that we have, let's just copy it, and drop down here, going to paste it in again, but this time, instead of is equal, let's make it is exact. And instead of a double equals, we're going to use a triple equals, so is it exactly the same.
We're not saying, is it roughly the same, is it exact. And then, for each one of these, let's just paste that in, all the way down the line. There we go. Now let's go back over, let's put a br tag here, just so we can see the difference. Br in the middle Let's go back over and reload our page. Every single one of those returns false. So, you see the importance of the triple equals here. If we have two types that are different, that may be different, then we want to use that triple equals. The other solution, that instead of letting PHP doing type juggling for you, you could do type switching and switch it yourself.
So you can make sure that whatever you're comparing gets converted to be two strings or to be two numbers. If you do that though make sure that you are certain that both types end up being the same thing. Otherwise PHP's type juggling is still going to kick in. So you might be setting something to be a string but if what you're comparing it to is an integer. Well PHP is going to go ahead and do it's switching behind the scenes. So spend some time playing with these false positives so that you understand what they are. But it also has an implication for another PHP function that's very common and that's empty. With the empty function, an empty string, zero, zero inside a string, null, false, and an empty array, are all considered to be empty. And that might not be what you would expect, especially that third one there, that third one really could trip you up.
You think, well wait a minute. I sent a string with zero in it back. That's not empty, that's zero. That's the number zero that I was trying to return to the user. Imagine that you have a web form that asks, how many children do you have? It wouldn't be very useful if we checked to see if they submitted a value for that form. But they're not allowed to put in the value zero. For zero children. We require them to have one, two, three or more children. So you can see how this would trip you up. To see this, let's close up our false positives page. Just save it and then close it. And let's go back to the page we were on before.
Which was our validations dot php. So it's down here, validations.php. And up at the top of it, you can see here's where we used empty. So let's just try some of these now. So if we have an empty string, well, then we know it's going to be empty, but let's try the number 0 in there. Let's reload our page, validation failed. See how that works? Put two 0's in there, now validation works. Let's put the number 1 in there, the validation works. But the 0 is a problem for us. So in addition to the example I gave where you might want to enter the number of children being 0.
It also comes up when you are working with select options, where one of the selections might be the number zero. It's important for radio buttons where you are choosing between true and false, like visible. A lot of times you'll send zero for not visible and one, true, if it is visible. It also comes up with checkboxes, because checkboxes often send zero if they're not checked and one if they are checked. Let's try another example here, though. Let's put an empty space in here. Now we've just got a space, in fact, let's put several spaces. Save it Let's reload the page.
It passes our validations. Says it wasn't empty. Now some of this is deciding what you mean empty is. But probably if someone had a web form and they just typed some spaces, you probably don't want to accept that value. We probably want to require them to put something in that field. Now dealing with spaces one way to do that is just to use the Trim function. So we'll trim all of the spaces out of there, leading and trailing spaces and see what we're left with. And if that turns out to be empty then we'll know that the field was empty, so that's a good easy fix. Another way we could do this is add something to this to say that well if it's not empty, and if it's not is numeric value. So if the value is not numeric, then we're going to allow it. So let's try that, let's try if we have the number 0 now. It passes our validations.
Another way to do it is not to use empty at all, but to just say, well the value must be exactly equal to this, an empty string. So after we trim it, if the value is exactly equal to that, Whatever they sent us, then we know that it's empty. So let's try that. Once again it a passes validations because we have a 0 in there. take the 0 out and now it fails our validations. Another commons solution that many developers adopt is to write their own version of empty which meets their specific needs. So you write your own function that says if it is one of the following values, and you check to see and you just get to pick from this list and decide...
Which things you think ought to be considered empty. If you don't think false should be considered empty, well then you can leave it out of your set. If you don't thing that zero is a string should be empty, you can leave that out of your set. So you can write your own custom function. Last bit of caution I want to give you about the logic of your validations is just to make sure you are careful with the basic operators as well. So many bugs get caused by using less than, less than or equal to, greater than, greater than or equal to Incorrectly. So you want something to only be five characters? Well, should you use less than or less than or equal to? You have to be careful about that and then the operators and, and or making sure that your logic really is sound between all those.
A lot of times it takes a little while to get it all worked out exactly. So that your conditions, when grouped, there are several of them, All still result in one true or false that has the correct answer you want.
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