Conditional styles

You can use PHP to control the way a page looks, by changing the styles it uses.

How many dogs?

Try this link to a PHP page that evaluates the number of dogs you have. Number of dogs is passed in as a URL parameter names dogs. Here, there are no dogs.

http://webappexamples.skilling.us/html/conditional-styles/how-many-dogs.php?dogs=0

Try a link for one dog:

http://webappexamples.skilling.us/html/conditional-styles/how-many-dogs.php?dogs=1

Now for two dogs:

http://webappexamples.skilling.us/html/conditional-styles/how-many-dogs.php?dogs=2

Finally, three dogs.

http://webappexamples.skilling.us/html/conditional-styles/how-many-dogs.php?dogs=3

What's the HTML?

Check the HTML the program makes when there are no dogs:

Now when there are two dogs:

The class is argh in one case, and good in another.

if

Here's the code:

Line 2 gets the number of dogs from the URL:

line 3 is:

That's a comment. Comments are Good Things.

Let's skip down a bit, and look at the output. Line 33 outputs the number of dogs:

The \n thing on the end adds a new line in the HTML, like hitting the Enter key. It doesn't affect what the user sees, but it does affect the HTML. Here's the HTML without the \n...

... and with it...

As you can see, adding the new-line thing makes the HTML easier to understand. That's particularly useful when debugging.

\n only works with you use double quotes (").

Works: print "<p>You rock!</p>\n";

Doesn't work: print '<p>You rock!</p>\n';

Line 34 is the main output:

That <?= $variable ?> is shorthand for <?php print $variable; ?>.

Notice how simple that code is. It outputs two variables: $class, and $message.

The statement prints some HTML. Here's what it prints, for one doggo.

$class is "fair", and $message is "You have a doggo! That's good, but you need one more."

Here's what it prints, for two doggos:

In that case, $class is "good", and $message is "You have two doggos! That's the perfect number!"

The program works by setting $class and $message to the right values. Then, when it gets to the output section, the code to use those variables is simple:

Most of the program's logic is working out the two variables. We put that code at the top of the PHP file. That's common. PHP programs usually look like this:

We do most of the work at the top, so the output can be simple. The top bit fills some variables, that are just output at the bottom.

Here's the code that sets the variables:

First, we make the variables, and set them to empty strings.

You don't have to do this; the program would work just fine without these lines. I added the lines as documentation, a reminder to me that the program works by setting the values of these variables. It also gives me a place to hang comments, if I wanted.

Here's the first if:

There's a test in parentheses (the () things). It's either true, or false. (There's a bit more to it. Later.) The parens are not optional. They must be there.

If the test is true, the code between the braces ({}) runs. If the test is false, the code is skipped. So the close brace (}) acts like EndIf in VBA.

Right!

The next code is:

No! It's right as it is. In VBA, we might write the code like this:

So = has two meanings in VBA. In an If, = means "compare." In the lines below the If, = means "assign," that is, take the thing on the right, and put it into the thing on the left.

PHP isn't like that. "Compare to" and "assign" are written differently. Use == for "compare to." That will be true or false. Use = for assign.

PHP would put 1 into $dogs, overwriting what was there! You'd get the wrong output, and lose the value of $dog as well.

Not good.

When there's only one case left, we can leave off the if.

Overall, the pattern is something like:

Another example:

The same thing:

In the second one, the variable's initial value is set to one of the options. That value is replaced in the if, if the condition is met.

Another example:

In the second line, the variable $state is trimmed of spaces, and converted to lowercase, before it is tested. We did the same thing in VBA.

If $state is invalid, the code shows an error message in the HTML, and stops. exit() is the same as End in VBA. We'll improve validation later.

It's a pattern

This...

... is common in programming, in any language. You can use it again and again, in different ways. Let's call it the multiway if pattern.

It goes into the pattern catalog. The catalog is a list of patterns you can reuse in your own code. If you're writing a program, and aren't sure what to do, check out the catalog. There's a link to it under Lists in the main menu.

Indenting and braces

We use some coding standards in the course.

Code in ifs and loops must be indented, by 2 or 4 spaces. Either is fine. You must be consistent within each file, though. Like this:

Also, don't leave out the braces. This would work...

... but don't do it! You're asking for bugs. Always put the braces in.

Check the coding standards page for all the deets.

Test for missing GET parameters

Here's some code from before:

What if you don't give dogs, or misspell it?

You'll get an error:

If a non-geek sees that, their head might explode.

Let's make a more friendly error message, like this:

It's still an error, but the message won't freak out the norms.

Here's some code. There are a few things going on, so pay attention to it. I'm just showing the new stuff.

Line 2 makes a variable called $errorMessage, and makes it MT. We'll use $errorMessage for two purposes:

• Hold error messages.
• Show whether there were any errors.

We'll use $errorMessage as a flag. If it's MT, there have been no errors. If it isn't MT, there have been errors.

Here a use example:

OK, back to the code:

Line 3 uses the isset() function. It's true of the parameter exists, false if it does not. If there's nothing in GET with the index dogs, we set $errorMessage to..., well, an error message.

Line 6 tests whether there are no errors. I could have used an else:

Instead of:

I used second if, since you'll be seeing that a lot. It's a quick way of testing whether any errors have happened.

Here's the code again:

Line 14 is down in the HTML output area. If there's an error message, show it. If not, do the rest of the output.

We'll use $errorMessage this way throughout the course. It's handy pattern.

Up next

Let's switch gears a bit, and see how PHPStorm helps use write and debug programs.