Right now all our media categories link to the same catalog.php page. 0:00 We want to be able to specify which category the catalog should display. 0:05 We can accomplish that by adding a string at the end of the URL. 0:11 After the file name, type a question mark, and a variable with any name. 0:14 Let's call it cat, short for category. 0:18 We use an equal sign, and then specify a value of books. 0:21 Variables added to the URL in this manner get sent to the server, 0:26 along with the request for the webpage. 0:30 They are available in a special php variable called _GET, 0:32 all uppercase, and our element cat in square brackets. 0:36 The element cat is the same name we gave our variable in the URL. 0:42 We can use this value in a conditional to check which category should be displayed. 0:46 There are four options for this page. 0:52 Our three categories, books, movies, and music, and the full catalog. 0:54 We want to check if the get variable cat 0:59 is set to one of these three specific categories. 1:02 And if not, show the full catalog. 1:05 Now it is possible to type anything after cat equals in our address bar, so 1:08 we want to make sure that we are only looking for one of the 3 categories. 1:13 We're going to edit our catalog.php file. 1:18 The page title with the value Full Catalog will be our default. 1:21 A default is used if none of the conditions are met. 1:26 In other words, if there's no variable set for the cat variable in the query string, 1:28 we'll display the full catalog, all the books, movies, and music in the library. 1:34 So let's write a conditional right after our page title that checks the value of 1:39 the get variable cat. 1:43 If, get, cat, 1:47 double equals, books. 1:52 We want to set our pageTitle equal "books". 1:58 To check for our other two categories we can add an else statement with another if. 2:05 Else, if, get cat 2:09 double equals movies 2:13 We'll set our $pageTtitle = "Movies". 2:25 Finally, else if 2:33 ($_GET["cat == music. 2:37 We'll set our page title = music. 2:45 Close out our conditional. 2:53 To make this a little easier to see when we click the links, 2:55 we're going to set the H1 tag to use our page title as well. 2:57 Open php echo $pageTitle. 3:00 Close php and save. 3:06 Now, we need to change your navigation to use these values. 3:09 Open header.php and add our query string after catalog 3:12 ?cat=books and for 3:18 movies we'll do ?cat=movies. 3:23 For music, ?cat=music. 3:28 Again, we can see that having an include file makes it so 3:35 much easier to make changes to our header. 3:38 Now when we go back into the browser. 3:40 And we refresh our page. 3:43 Our navigation will pass the correct variables. 3:45 Books, movies, music. 3:49 And the title is displayed on the page based on what category we're in. 3:52 One last thing we should do in this conditional. 3:57 We're currently checking the value of the get variable cat here. 3:59 But, if the variable doesn't exist at all, 4:02 then this conditional would generate a notice. 4:05 Let me show you. 4:07 Let's remove the cat equals music. 4:09 Yuck. 4:14 Not something we wanna see. 4:15 Let's fix that. 4:16 Let's go back into work spaces, and our catalog file. 4:18 Before this conditional, we need to do another check. 4:22 We're going to use php's isset function. 4:26 If (isset ($_GET["cat"] 4:29 Then move on to execute. 4:42 Is conditional. 4:43 Now when we save this file and go back into our browser, 4:49 we refresh the page and we see our full catalog, books, movies, and music. 4:53 We still need to actually display the full catalog or 5:01 the categories which we'll do a little later on. 5:03 We have everything in place to choose the category we want to show. 5:06