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When we accept data from users especially from web forms, let's say, submit, we almost never want to just accept any old data that they send In fact, it's to our benefit as developers to assume that users are sending us the worst kinds of data possible, that may be even trying to hack our site and do us harm. Checking the submitted data carefully is the road to having robust and secure code. So we need to spend some time thinking about requirements for our data and learning how to enforce those requirements. I promise you're going to spend significant time on this for every project that you do, so it's worth investing the time in learning how to do it and to avoid the pitfalls. Imposing data requirements is called validating our data, and you'll hear me refer to the process as validations or passing or failing our validations. If data passes validations, it means that data was acceptable and we can use it. If it fails validations, that means there was a problem and we need to reject it. And most often that means going back to the user to ask them to make revisions and submit it again.
The simplest and most common requirement for a form field is that the user submits some value that the form field can not be left blank. We call this validating the presence making sure that something was present in the field. Now we can have other requirements too. For example, we could check for the number of characters that they submitted to make sure that something is either longer than a certain number of characters, shorter than the certain number, maybe between a certain range where that is exactly a certain number of characters. We can validate the type, make sure that they've sent us a string or an integer or a float, if that's what we're expecting. We can validate that it's included in a set from a select set of choices. So, if we'd ask someone to choose whether they're male or female, the answer we get back, we would expect to be either male or female. And if it's not one of those two choices, we want to reject it. Once we start working with databases, we'll start wanting to check whether things are unique or not.
This especially comes into play with something like a username. Everyone needs to pick a unique username, so we need to take the value they've given us for their preferred username and then check the database to see whether or not that is a valid, unique username. And the last common one would be format. And that's just checking that an email has the at symbol in it, that currency has a dollar sign at the beginning. For dates and times they might need to end with a.m. or p.m. Any kind of format for the format that something should be submitted to us in, we can also check for. Now these are general categories.
They're, by no means, a full list. You'll need to write custom validations. For example, a field might contain a path to a PDF file and your validation would check to make sure that that file already exists. Or you might validate whether a value is greater than 20 or that a value is odd or even. Really, the sky's the limit when it comes to validations. So let's try a few of these common ones just to get a feel for them. So to start with, let's open up basic.html again and we'll do Save As on it. I'm going to call it validations.php. Validations. So in here let's open our PHP tags and I'm going to put some place holders for the different types that we just talked about are common.
Present string length, type inclusion, uniqueness, and format. So let's go through each one of these. Let's talk about presence first. So, with presence, you could use isset. We can say, just make a conditional if it's not set. And let's say we're just going to check for value. Then, we can check and say well, the validation failed. Echo validation failed. Okay. So that just checks whether the value was actually set. So let's say, we have value equals quote, quote, semicolon. Right? So we have an empty string sent. Well, it is set.
That's not what we were checking for. It's good to check to see if it's set or not, that's a good sanity check. But a blank field will still set a key in our post superglobal. So, it's not a bad fail-safe to have, but we really need to do more checking than that. And one way to do that is with empty. And I'll just do empty value. So, now if we try this. Let's bring it up in a browser. Let's check validations.php. The validation failed but, if we have something in there let's put an x. Save it. Reload the page, and it succeeded. We didn't get anything back.
So we're only getting a message if it fails our validations. So I'm going to make it so that it does pass. We did make sure we have the presence there. Incidentally, we're going to talk more about empty in the next video and some of the problems that come up with it. So, for now, we're going to leave it. And just make that what we look at for presence. We'll revisit it in a moment. With string length, we could have the minimum length. Let's say that we have another value here, and for now, let's say that the value is empty again. And this time, we're going to check to see if the string, we know it's present. Now we want to see string length, strlen, of value.
And that's going to return an integer for how large it is. Is it less than the minimum? And if it is, well, then the validations failed. I'll just copy that and paste it down here. So that's going to be minimum length and that's minimum length. And let's set a value for the minimum. Let's say that the minimum is equal to 3. So it must be at least 3 or it fails our validations. Let's go ahead and make another one here and we'll just do max length.
And we'll use the same value but we're going to write max equals, and let's make it 6. So if it's greater than the max, well, then we have another problem. So, we're validating the length is, above a minimum and below a maximum. So, let's just try that real quick. We've got this value here, that's empty right now. Let's reload the page and validation failed. That's not my first validation. This one's passing. It's one of these that we're seeing. And it doesn't tell us which, but that's okay. We know which one it is. Let's go ahead and make this value, now, something else. Let's say that it's abcd.
So now it's in that range and it works just fine. We make it efghijk, save it, validation failed. Now, of course, you could use custom messages for this, so that it would say that the validation failed because of the following reasons. But for right now, I just want to show you how validations work. So let's do type. I'm just going to copy this because it's going to be very similar. And let me do value equals first and this time the test though that we're going to put in here is going to be, is it a string? So, if value is not a string, then the validation fails.
So, if we put in here a number. Let's put 1. It comes back just fine. But if we put in 1 as an actual integer, then it doesn't. Now this brings up an important point, that when we submit form values in PHP, there always going to be strings. Even if it's a number, it's going to be submitted as a string, because PHP doesn't know whether we intended it to be a 1 as a string or 1 as a number. So it's going to be a string. And you'll need to do a conversion to convert its type if you need it to be something else.
Let's do inclusion in a set. Let's take this right here. Now, let's say that we have a set of values here. Let's say that the set is equal to the array. And in the array, we have 1 or 2 or 3 or 4. So it must be in one of these values. So how do we do that? Well we know we can use the in array. You want to check if it's not in the array. And so for that we use value followed by the set. So if value is in the set it will pass. If it's not then it won't.
Let's try that real quick. Okay. It passed. We now make it 5 and now it fails our validation. You getting a feel for how this works? So uniqueness, we really can't do without databases. So we're going to leave that one for now. But the basic idea is that when they submit a value, we then turn right around and take that value and ask the database. Hey, database. Do you have this value? And if it returns yes it does, well, then, we erase the validation error. If it returns no it doesn't, then we don't. And then last of all, I want us to look at format. Now, for format, I want to teach you a new function.
It's a little bit of a high-level function, but it's super-useful. And it's preg_match. And what we're doing is we're applying a regular expression to see if something matches. So here's the format. We provide a regular expression here and the subject we want to match, and it returns true whether it matches or not. Let me just give you a quick example here. I'll just paste it in, so here's our regular expression inside the slanted lines as a string, and I'm going to match whether PHP is inside PHP is fun. And it will either say match was found or match was not found.
Switch back over here, reload it, a match was found and if we put in something else like an x at the end. Then it comes up and says that a match was not found. So that's how it works. Let's instantly just got back up here and fix this one just so that passes it's validation. And I'm just going to make a note here, uses a database to check uniqueness. So let's just drop back down here. And let's finish working on format. So the way that we're going to do this, is very similar to what we've been doing. We'll write an if statement. But instead of in_array, were going to use this preg_match. There we go, I'll drop it in.
But we don't want to check that. Were going to check it against some value. Let's say value is equal to email@example.com, and you can write a complex regular expression here for now. I'm just going to do, real simple, let's check to make sure that there is an at symbol in it, and we're going to use value. That's all it's going to do, is check to see whether the at symbol is inside there. So let's go back here. Oops, unexpected if. Where did I get my problem? I forgot my semicolon. There we are. Back, there we go. Match validation failed. Why did it come back and tell me that it's not there? It should be, if it's not matching.
That was my mistake. So, not match. So, it did match, so now if it does not match, then we want to return validation failed. Now we don't get our validation error. Now, preg_match is great because you can use regular expression which are very powerful. I actually have a regular expression training on lynda.com that you can refer to that'll tell you more about how to use regular expressions. But I think that it's not the fastest, sometimes using string, string, STR, STR. And string pos for postion, STR POS, are going to actually be a little bit faster.
So for example, you can have something like this, if the string position of the value is exactly equal to false then the validation failed. Now why did I use exactly equal? Because string position will return 0 if it finds it at the beginning of the string. And so we don't want to have equals false because 0 would be considered false in that case. So if it's exactly equal to false using that triple equals, then we want to match it... And that leads us into a discussion then, about some of the problems with validation logic. Because those kinds of problems can trip you up and give you false positives or false negatives.
So, let's look at that in the next movie and talk about what some of the common problems are.
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