Overview of This Lesson

Refresh this page because I am probably still making changes to it.

In this lesson we'll learn about PHP and what it does, a brief history of PHP's development, and how to code with PHP. You should make note of the following resources:

Resources

The following references from the PHP Manual will be helpful:

Pre-Requisites

Before doing these tutorials, it's important that you already have the skills to code in HTML and that you have basic programming experience. It's also recommended that you review the basics of the client-server model an request/response cycle.

What is PHP?

PHP, a recursive acronym that stands for "PHP: Hypertext Preprocessor", was born in 1995 from a set of tools developed by a Danish-Canadian software developer named Rasmus Lerdorf. He used Perl/CGI to create some scripts that kept track of visitors to his on-line resumé - basically a guest book for visitors to sign and a counter that displayed the number of visits to the page. This was a new idea in the 90's and this technology had not previously existed, so many other web developers were interested in Lerdorf's tools. He packaged them together as "Personal Home Page tools" and gave them freely to anyone else who wanted to use them. I've been around computers and web development enough to remember these tools, and how exciting it was to have that little visitor counter at the bottom of the page!

one version of my old web site with a visitor counter
A very early Terminal Learning archived on Wayback Machine showing the visitor counter at the bottom of the page.

Over a couple of years, Lerdorf continued to add on to his tools (although he switched from Perl to C), including some code that would process form data. This became PHP-FI (Personal Home Page - Form Interpreter). Lerdorf was later joined by developers Zeev Suraski and Andi Gutmans, and PHP 3.0 was released in 1998 (at this time, the name was changed to PHP: Hypertext Preprocessor).

In 2000, PHP 4.0 was released with a new parser (part of the compiler) called the Zend Engine, developed by Suraski and Gutmans, who founded Zend Technologies in 1997 (today they are simply called "Zend"). Zend is now responsible for developing tools for developers of PHP programs. PHP 4.0 included some object-oriented functionality, better encryption algorithms, support for Java object binding, and a powerful regular expression library. PHP 4.0 also performed much more efficiently than previous versions of PHP, which only helped to increase its popularity.

PHP 5.0 was released in 2004 with many new features and improvements, including much more support for object-oriented coding, exception handling, better string processing capabilities, and support for XML and web services.

PHP 6.0 was never released: it was supposed to include better support for multi-language and unicode characters, however several delays and problems resulted in bits and pieces of it being released in later versions of PHP 5. Eventually PHP release 6.0 was abandoned.

PHP 7.0 was released in 2015 and included several performance improvements and some new language features, mostly in how function parameters and return types are defined/declared.

PHP 8.0 was released in in 2020 and is the current major version of PHP; the current version is 8.1, although 8.2 is slated for release in November 2022. PHP 8.x has some significant changes to previous iterations of PHP such as JIT compiling, match expressions, enumerations, changes to data types, and some changes to syntax.

Why use PHP?

PHP is a server-side language, so it can be used in addition to client-side technologies like JavaScript to perform tasks on the server side. Key points about PHP:

Advantages and Strengths of PHP:

How Write PHP Programs

Let's jump right in with a demonstration: Create a new project and add a file called index.php. Add this simple line of code to your file and then save the file. In a moment, we'll talk about what this statement does, don't worry. Note that all your PHP programs should have the .php extension (even though they will often contain HTML also, they should still have the .php extension). 

<?php phpinfo(); ?>

PHP is a server-side language: it is executed on the server, not on the client machine (i.e. not in the browser). When you code a PHP program, you have to upload your code to the server. Unlike client-side code such as HTML or JavaScript, you can't test or run your PHP by opening the file in the browser... if you do that, you'll only see PHP code! Your browser does not have a compiler or a virtual machine that compiles and runs PHP, so it treats your file as plain text.

You can try it if you want to: open your file in your preferred browser - you'll simply see your code. Note that if you right-click and Inspect your page, you'll still see some of the minimal HTML because your browser adds minimal DOM.

php file won't run if you load it directly into a browser
If you load a PHP file into a browser, you'll only see the code.

To run a PHP file, you must upload it to a server. Assuming you have access to a web server that runs PHP, you then only need to use a regular URL to run your program. For example, https://terminallearning.com/devPhp/sample.php loads and executes a file called sample.php that displays "Hello, World!"

If you load the above sample PHP file, and then right-click/Inspect it, you won't see any actual PHP code. That's because the PHP code is executed on the server: when your browser requests the PHP file, the server receives the request, finds the file, and then executes all the PHP code inside the file. The output of the PHP program is plain text and HTML code. That output is then send as the response back to the client machine. The client machine retrieves the response and renders it in the browser.

When you load your index.php file from its URL, you'll see a huge amount of information in nicely formatted tables: all this output from one line of code!

information about the PHP installation in the browser
Upload your PHP file to a server and then load it in the browser via its URL.

What you're looking at is information about the PHP installation on your server: This is a list of all the properties and their values. This page is handy to have when you are working with a server that's not your own: you can load phpinfo() whenever you want to need to answer questions like, "What version of PHP am I using?", "Can I use HTTPS?", and "What is the context root of this project directory?"

Structure of a PHP File

As I've already hinted, we often mix our PHP code in with our HTML code. You've probably learned with JavaScript that it's not a good practice to mix your JavaScript code with your HTML because it doesn't lend itself to good program design (e.g. SOLID principles, progressive enhancement). With PHP, this is not an issue. PHP code is never compiled and run in the browser so it will not interfere with specialized devices and screen readers that can only read text/HTML. A PHP web application returns plain text and HTML back to the browser, which is supported by specialized devices and screen readers.

When you insert PHP anywhere inside an HTML document, you use special tags. These tags communicate to the server that the contents of the tags contain PHP code and should be parsed and executed before the page is sent along in the response back to the client.

There are three different PHP tags:

This means you can easily mix your PHP and HTML, which is a very common way of coding PHP pages: 

<!doctype html>
<?php 
  // get the pet name from the name input field
  $petName = isset($_POST["petName"]) ? htmlspecialchars($_POST["petName"]) : "";
  $record = getPatientRecord($petName, $_SESSION("custId");
?>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Sydney Animal Hospital
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <script src="scripts/pageComponents.js"></script>
    <script src="scripts/forms.js"></script>
    <link rel="stylesheet" href="styles/main.css">
    <link rel="stylesheet" href="styles/dynamic.css">
  </head>
  
  <body>
    <!-- Pet name in page header -->
    <header>
      <h1>Sydney Animal Hospital</h1>
      <h2><?= $petName ?> Patient Record</h2>
    </header>
                        
    <!-- outstanding balance notice, if applicable -->
    <? if ($record['balance'] > 0) {
      echo "<p class='notice'>You have an outstanding balance of ".
              $record["balance"]."</p>";
    }?>
                    
    <! -- rest of page content here -->
                        
    <footer>
      <address>&copy; 2017 Sydney Animal Hospital</address>
      <!-- navigation filled via JavaScript -->
      <nav></nav>
    </footer>
  </body>
</html>

Notice the different areas where the PHP code has been embedded within the HTML code. The actual details of the code aren't Important yet, but you're probably curious so here are some general details.

  1. At the top of the document: This code sets the values of a couple of variables: one comes from some form input and the other comes from a function (the function is accepting a session variable as one of its arguments). There are two full, executing statements so they go inside a standard <?php ?> tag, although you could have used the shortened version <? ?> if the server supports it.
  2. In the <header> element: This quick PHP echo tag inserts the contents of a $petName variable in front of " Patient Record" inside the <h2> element. This is a simple expression so it can be placed inside the <?= ?> tag. This is equivalent to the code <?php echo $petName; ?>.
  3. Under the <header> element is a block of code using the short php tag <? ?>. You could have also used <?php ?> if your server doesn't support the shorter tag (the server this page resided on supports the short version). This segment of code is an if statement that prints a notice about the customer's outstanding balance if that balance amount is more than 0 dollars. The variable $record is an associative array or hash array, where the indexes are unique strings. Likely, this $record array was populated from a database query or a data file of patient information.

You can easily see that mixing your PHP code within your HTML code does make parts of the page harder to read, so make sure you use caution and pay attention to details when you are coding with PHP.

Mixing PHP with your HTML code does not interfere with accessibility and progressive enhancement: PHP is executed on the server side and the response that is returned to the client is still in plain text and HTML, with no actual PHP embedded within the HTML code. It is a common practice to mix PHP and HTML this way, although may developers who are more familiar with Soc and patterns like MVC will want to find a way to have more separation of PHP and HTML.

This can easily be done by using your HTML file to create a template or "view", and then using your PHP file to contain the variables and other data, functions, logic, etc. Here is a very basic example (and I mean really basic, we can see more examples as we learn more about PHP):

First you start with an HTML template file. This is just HTML code with PHP tags where you want dynamic content to go. 

<!doctype html>
<html lang="en">
    
    <head>
        <meta charset="utf-8">
        <title>Sydney Animal Hospital</title>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <script src="scripts/main.js"></script>
        <link rel="stylesheet" href="styles/main.css">
    </head>
    
    <body>
        <header>
            <h1>Sydney Animal Hospital</h1>
            <h2><?= $petName ?> Patient Record</h2>
        </header>
        
        <!-- display table of pet information -->
        <?= getPetInformation($record) ?>
    
        <! -- rest of page content here -->
        
        <footer>
            <address>&copy; 2017 Sydney Animal Hospital</address>
            <!-- navigation filled via JavaScript -->
            <nav></nav>
        </footer>
    </body>
</html>
index.template.html

Then you create your PHP file, which contains constants, variables, functions/methods, and all the stuff that will contain/return the values you want to display in the template page: 

<?php 
    ini_set('display_errors', 1);
    ini_set('display_startup_errors', 1);
    error_reporting(E_ALL);
	
	// get the pet name from the name input field
	$petName = isset($_POST["petName"]) ? htmlspecialchars($_POST["petName"]) : "";
	$record = getPatientRecord($petName, $_SESSION("custId");
	
	function getPetInformation($record)
		/*  code that creates HTML elements containing information 
		    about the pet patient */
		return htmlText;
	}
	
	require("index.template.html");	
?>
index.php

Notice that index.php is our main page. At the end of the code, you can see the require() function. This is one of several functions you can use to load any file into another file. It's a lot like an include: this statement copies the contents of the index.template.html file and places it into the index.php file. Remember that index.php will be the file that is requested by the browser: when the server processes the request, it will execute all the PHP code and load all the functions. Then it will retrieve the index.template.html file and "fill in the blanks", or fill all the PHP tags with their appropriate values from the PHP that was already executed.

There are four methods you can use to include a file in a PHP file:

In my example I chose require() because the index.php page is useless without the index.template.html file, so if there's a problem loading the template, we want to know about it. For this simple example, it probably doesn't matter if you choose require() or require_once() but I suspect that if the user refreshes or reloads the page, we might like to re-load the template from the server, just in case.

Debugging PHP Programs

If you are encountering errors and problems with your PHP programs, you'll find that the browser output is not very helpful. For example, this code has an error (you'll learn more about this error in an upcoming lesson): 

<?php 
    echo "Hello, World!";  // "echo" means print to the document
    $foo = "bar";  // declaring and initializing variable
    echo foo;  // error: missing the $ in front of foo
?>

But when you load the page from the server, you don't see any error messages, just some output that you weren't expecting:

the page says Hello, World!foo
There are no error messages.

When you're debugging, you can add a few extra lines of code to show helpful error messages when an error or exception occurs: 

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

The ini_set(configOption, value) method sets the value of a specific configuration option. The display_errors option set to a value of 1 or true will turn on the display of errors on the page instead of hiding them from the user. The display_startup_errors option set to a value of 1 or true will turn on the display of any errors/exceptions that occur when the PHP code starts: normally these errors won't appear with only display_errors set.

The error_reporting(E_ALL) method displays parsing errors on the page. Normally the first two settings won't display compilation or syntax errors, but this last line of code will ensure those errors appear on the page.

Adding these lines of code to the program, I now get more helpful error messages on the screen: 

<?php 
  ini_set('display_errors', 1);
  ini_set('display_startup_errors', 1);
  error_reporting(E_ALL);

  echo "Hello, World!";  // "echo" means print to the document
  $foo = "bar";  // declaring and initializing variable
  echo foo;  // error: missing the $ in front of foo
?>

Now that you've learned what PHP is and how it works, it's time to start learning the PHP syntax so you can write your own server-side PHP programs!

Practice Quiz

Try this quiz to see how well you remember what you've been learning.

  1. Read the question and choose an answer.
  2. Click the Check Answer button to check your answer.
    • If the answer is wrong, you can try a different answer.
    • Answers you've already selected are highlighted.
  3. Once you find the correct answer, or if you just want to move on, click the Next button.
  4. After the last question, you can start the quiz over if you want.

The questions and answers are randomized, so that you are encouraged to use your critical thinking skills.