PHP

Overview PHP is a server-side scripting language that is primarily used to make dynamic websites.

Can you do with PHP?

Facebook, Canva, Slack, Wikipedia are all (originally) written with PHP
Web scraping
Connect to MySQL database
Install Wordpress

Hello World

PHP is a non-compiled language and therefore your machine (personal computer or a web server) must have PHP installed to execute the scripts. To see if you have it installed, and learn about the PHP version and modules you have, create a file named phpinfo.php with the following and place it in the directory where your website files live.

<?php
    phpinfo();
?>

GET Variables

We organize data and functions of a website through its pages. For example, if you display a list of courses on courses.php and want to let the user click on a course to edit on edit.php then you will need to send data between the pages so edit.php knows which record to query.

A good way to send data between pages is to pass a GET variable in the url string. In the above example this might look like this: edit.php?id=3 where 3 is the id of the record in the database. This method is also good because it consistently returns the resource, regardless of whether or not the user came from the first page. For example, this url will always display a google search for “cats”: https://www.google.com/search?q=cats

<!-- courses.php -->
<a href="edit.php?id=<?php echo $id; ?>">Edit</a>

<?php
$id = $_GET["id"];
// connect to mysqli
// create and run selection
$sql = "SELECT * FROM courses WHERE id='$id'";
$result = $conn->query($sql);
?>

Filtering results

Another good use for passing data between pages is to filter the data shown. In the courses example, say you want to let the user show only courses in a particular department. To do this you could add an HTML select to the top of courses.php that, once selected, uses Javascript to reload the current page with a GET variable like courses.php?deptid=5. Then, at the top of courses.php add an if statement that checks for the GET variable, and if there is a department then append additional parameters to the query string before you write the data on the page. For example:

<?php
$sql = "SELECT * FROM courses";
if (isset($_GET['d'])) {
    $sql .= " WHERE department='" . (int)$_GET['d'];
}
$sql .= ";"
...

Note, you could use the department name instead of the id, but spaces look gross in a URL field and can be unpredictable courses.php?d=digital%20studies. The professional solution is to use a lowercase string with hyphens and url rewriting in an .htaccess file to allow a url string like courses/digital-studies. This makes the url human-readable and hides the technology you are using.

For example, while both of these urls work, the second one hides the p variable and makes it much cleaner:

Use GET to increment a value

Another common pattern with PHP is to pass variables using GET to a different page that handles the database update and redirects back to the original page.

<!-- courses.php -->
<a href="rate.php?id=321&rating=1">Up button</a>

<?php
// 1. make sure the values are what you expect first, then...
// 2. update database using id and rating from GET variables
// 3. redirect back to the original page
header("Location: courses.php");
die();

References

W3Schools PHP

PHP