- How PHP works.
- To write a simple PHP page.
- To understand and work with simple PHP variables.
- To use PHP operators.
- To pass values from one page to another via the URL.
How PHP Works
When a user navigates in her browser to a page that ends with a .php extension, the request is sent to a web server, which directs the request to the PHP interpreter.
As shown in the diagram above, the PHP interpreter processes the page, communicating with file systems, databases, and email servers as necessary, and then delivers a web page to the web server to return to the browser.
The php.ini File
Before we look at PHP syntax, we should briefly mention the php.ini file. This is a plain text file that is used to configure PHP. When the PHP interpreter is started, it reads the php.ini file to determine what settings to use. We will mention this file from time to time throughout the course, but for now, it is enough that you are aware of its existence.
Basic PHP Syntax
PHP code must be contained in special tags so that the PHP interpreter can identify it. Depending on the PHP configuration, these tags can take several forms:
<?php PHP CODE GOES IN HERE ?>
|This is the most commonly used (and recommended) form. It is known as the XML style, because it can be used inside of an XML document without causing the document to become poorly formed.|
<script language="php"> PHP CODE GOES IN HERE </script>
|HTML or Script style tags.|
<? PHP CODE GOES HERE ?>
|“Short” tags. (see footnote)|
<% PHP CODE GOES HERE %>
|ASP-style tags. (see footnote)|
In this manual, we will use the first form shown as it is the most common and the most portable.
PHP Statements and Whitespace
PHP statements must be inside of PHP tags to be processed by the PHP interpreter. Each PHP statement must end with a semi-colon, which tells the PHP interpreter that the statement is complete. If a semi-colon does not appear at the end of a line, the interpreter will assume that the statement continues onto the next line.
The PHP interpreter condenses all sequential whitespace in PHP scripts to a single whitespace. This convenient feature allows PHP developers to structure their code in a readable format without being concerned about the effects of line breaks and tabs.
PHP has two forms of comments:
- Single-line comments begin with a double slash (//).
- Multi-line comments begin with “/*” and end with “*/”.
// This is a single-line comment /* This is a multi-line comment. */
There are literally hundreds of built-in PHP functions that do everything from returning the current date and time on the server to pulling data out of a database. A function might take zero arguments (e.g, phpinfo(), which returns information on the PHP environment) or it might take several arguments (e.g, mail(), which takes three required and two optional arguments). The syntax for calling a function is straightforward:
The example below shows how the phpinfo() function works.
Code Sample: PhpBasics/Demos/PhpInfo.php
<html> <head> <title>PHPINFO</title> </head> <body> <?php //Output information on the PHP environment phpinfo(); ?> </body> </html>
Introduction to php.net
PHP functions are well documented at http://www.php.net. You can quickly look up documentation on a function by going to http://www.php.net/function_name. For example, to see documentation on phpinfo(), go to http://www.php.net/phpinfo. (see footnote)
It is an unwritten rule that every programming course must contain a “Hello World!” script. Here it is:
Code Sample: PhpBasics/Demos/HelloWorld.php
<html> <head> <title>Hello World!</title> </head> <body> <?php //Write out Hello World! echo 'Hello World!'; ?> </body> </html>
Notice the following about the above code:
- Code between <?php and ?> is processed by the PHP interpreter.
- The echo command is used to print text back to the browser.
This code isn’t very exciting. In fact, PHP doesn’t buy us anything here as we could have just as easily output the result using straight HTML. There is nothing dynamic about this script. After learning about variables, we’ll take a look at some more interesting examples.
PHP variables begin with a dollar sign ($) as shown below.
$var_name = "Value";
|String||string of characters|
|Boolean||true or false|
|Array||list of items|
|Object||instance of a class|
Variable Names (Identifiers (see footnote))
- consist of letters, digits, underscores and dollar signs
- cannot begin with a digit
- are case sensitive
PHP is weakly typed, meaning that variables do not need to be assigned a type (e.g, Integer) at the time they are declared. Rather, the type of a PHP variable is determined by the value the variable holds and the way in which it is used.
Here is the “Hello World!” script again, but this time we use a variable.
Code Sample: PhpBasics/Demos/HelloVariables.php
<?php $Greeting = 'Hello World!'; ?> <html> <head> <title><?php echo $Greeting; ?></title> </head> <body> <?php echo $Greeting; ?> </body> </html>
This time the string “Hello World!” is stored in the $Greeting variable, which is output in the title and body of the page with an echo command.
Exercise: First PHP Script
In this exercise, you will write a simple PHP script from scratch. The script will declare a variable called $Today that stores the day of the week.
- Open a new document and save it as Today.php in the PhpBasics/Exercises folder.
- Declare a variable called $Today that holds the current day of the week as literal text.
- Output $Today in the title and body of the page.
- Test your solution in a browser. The resulting HTML page should look like this:
Instead of assigning a literal string (e.g, “Monday”) to $Today, use the built-in date() function so that the script won’t have to be manually updated every day to stay current. For documentation, visit http://www.php.net/date.
A variable’s scope determines the locations from which the variable can be accessed. PHP variables are either superglobal, global, or local.
|superglobal||Superglobal variables are predefined arrays, including $_POST and $_GET. They are accessible from anywhere on the page.|
|global||Global variables are visible throughout the script in which they are declared. However, they are not visible within functions in the script unless they are re-declared within the function as global variables.|
|function||Variables in the function scope are called local variables. Local variables are local to the function in which they are declared.|
Again, superglobal variables are predefined arrays, including $_POST and $_GET and are accessible from anywhere on the page. The complete list of superglobals is shown below.
- $_GET – variables passed into a page on the query string.
- $_POST – variables passed into a page through a form using the post method.
- $_SERVER – server environment variables (e.g, $_SERVER[‘HTTP_REFERER’] returns the URL of the referring page).
- $_COOKIE – cookie variables.
- $_FILES – variables containing information about uploaded files.
- $_ENV – PHP environment variables (e.g, $_ENV[‘HTTP_HOST’] returns the name of the host server. (see footnote)
- $_REQUEST – variables passed into a page through forms, the query string and cookies.
- $_SESSION – session variables.
The elements within superglobal variables can be accessed in three different ways, which the authors of PHP and MySQL Web Development refer to as short style, medium style, and long style. (see footnote)
|Style||Syntax (using $_GET)||Notes|
Many of these superglobals will be covered later in the course.
Constants are like variables except that, once assigned a value, they cannot be changed. Constants are created using the define() function and by convention (but not by rule) are in all uppercase letters. Constants can be accessed from anywhere on the page.
Variable-Testing and Manipulation Functions (see footnote)
PHP provides built-in functions for checking if a variable exists, checking if a variable holds a value, and removing a variable. (see footnote)
|isset()||Checks to see if a variable exists. Returns true or false.||isset($a)|
|unset()||Removes a variable from memory.||unset($a)|
|empty()||Checks to see if a variable contains a non-empty, non-false value.||empty($a)|
Operators in PHP are similar to those found in many modern C-like programming languages.
$a + $b
$a - $b
$a * $b
$a / $b
$a % $b
$a . $b 'Hello' . ' world!'
$a = 1; $c = 'Hello' . ' world!';
+= -= *= /= %= .=
$a += 1; $a -= 1; $a *= 2; $a /= 2; $a %= 2; $a .= ' world!';
|Increment By One||
|Decrement By One||
$foo = ($age >= 18) ? 'adult' : 'child';
$a = @(1/0);
Creating Dynamic Pages
Single Quotes vs. Double Quotes
In PHP, for simple strings you can use single quotes and double quotes interchangeably. However, there is one important difference of which you need to be aware. Text within single quotes will not be parsed for variables and escape sequences (see footnote). Compare the examples below.
Code Sample: PhpBasics/Demos/SingleQuotes.php
<html> <head> <title>Single Quotes</title> </head> <body> <?php $person = 'George'; echo '\tHello\n$person!!'; ?> </body> </html>
Because of the use of single quotes above, the string “\tHello\n$person!!” will be output literally, as shown below.
Code Sample: PhpBasics/Demos/DoubleQuotes.php
<html> <head> <title>Single Quotes</title> </head> <body> <?php $person = "George"; echo "\tHello\n$person!!"; ?> </body> </html>
This time, because of the double quotes, the string will be parsed for variables and special characters and will be output as shown below.
To see the effect of the special characters (\n and \t), you will have to view the source of the resulting page.
Passing Variables on the URL
A common way to pass values from the browser to the server is by appending them to the URL as follows:
The part of the URL that follows the question mark is called the query string. One or more name-value pairs can be passed to the server in this way. Each name-value pair is separated by an ampersand (&). The processing page can read these name-value pairs and use them to determine its response.
The HTML page below shows an example of how these name-value pairs might be passed.
Code Sample: PhpBasics/Demos/HelloHi.html
<html> <head> <title>Preferred Greeting</title> </head> <body> Do you prefer a formal greeting or an informal greeting? <ul> <li><a href="HelloHi.php?greet=Hello">Formal</a></li> <li><a href="HelloHi.php?greet=Hi">Informal</a></li> <li><a href="HelloHi.php?greet=Howdy">Friendly</a></li> </ul> </body> </html>
Code Sample: PhpBasics/Demos/HelloHi.php
<?php //Assign the passed variable to a variable with //a more convenient name. $greeting = $_GET['greet']; ?> <html> <head> <title><?= $greeting ?> World!</title> </head> <body> <?php echo "$greeting World!"; ?> </body> </html>
Notice the following about the code above.
- Variable names begin with a dollar sign ($).
- Values passed in the query string are part of the $_GET array and can be accessed using the following syntax: $_GET[‘fieldname’].
- A shortcut for echo ‘text to print’; is <?= ‘text to print’ ?>. (see footnote)
Exercise: Passing Variables via the Query String
In this exercise, you will write a script that says hello to different people based on what is passed through the query string.
- Open PhpBasics/Exercises/HelloWho.html in your editor. You will see that it is the same as the demo we looked at earlier.
- Modify HelloWho.html so that it has four links, each of which passes the name of one of the Beatles (Paul, John, George, and Ringo) to HelloWho.php, which is in the same directory.
- Open PhpBasics/Exercises/HelloWho.php in your editor. Modify the code so that it outputs a greeting based on the link clicked on HelloWho.html.
- Try it out in your browser.
Change the links so that each Beatle gets a custom greeting (e.g, Howdy Paul, Hi John, Bye George, Hey Ringo).
PHP Basics Conclusion
In this lesson of the PHP tutorial, you have learned how to write a simple PHP page, to recognize and look up documentation on PHP functions, to work with variables and to pass variables from one page to another via the query string.