In this lesson, we will introduce you to PHP and show you how it works. You will also be able to write your first PHP script after learning the basic syntax of PHP.
- Learn what PHP is.
- Learn how PHP works.
- Write a simple PHP page.
- Understand and work with simple PHP variables.
- Use PHP operators.
- 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. Must be enabled via the short_open_tag php.ini configuration file directive.|
<% PHP CODE GOES HERE %>
|ASP-style tags. Must be enabled via the asp_tags php.ini configuration file directive.|
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.
<!DOCTYPE HTML> <html> <head> <meta charset="UTF-8"> <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.
Another very good function reference is located at http://www.phpdig.net/ref.
It is an unwritten rule that every programming course must contain a "Hello World!" script. Here it is:
<!DOCTYPE HTML> <html> <head> <meta charset="UTF-8"> <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
?>is processed by the PHP interpreter.
echocommand 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.
$varName = "Value";
|String||string of characters|
|Boolean||true or false|
|Array||list of items|
|Object||instance of a class|
Variable Names (Identifiers)
Variable, function and class names are all identifiers and all follow the rules above, with the exception that function names are not case sensitive.
- 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.
<?php $greeting = 'Hello World!'; ?> <!DOCTYPE HTML> <html> <head> <meta charset="UTF-8"> <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
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
|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
$_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.Which environment variables are available depends on the specific server setup and configuration.
$_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.
PHP & MySQL Web Develpoment, Third Edition.
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
For a complete list of variable functions see http://www.php.net/manual/en/ref.var.php.
PHP provides built-in functions for checking if a variable exists, checking if a variable holds a value, and removing a variable.
||Checks to see if a variable exists. Returns true or false.||
||Removes a variable from memory.||
||Checks to see if a variable contains a non-empty, non-false value.||
Operators in PHP are similar to those found in many modern C-like programming languages.
$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||
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
Escape sequences are used for characters that cannot easily be output within strings. Common escape sequences are \n for a newline, \t for a tab, \\ for a backslash, \" for a double quote, and \$ for a dollar sign.
<!DOCTYPE HTML> <html> <head> <meta charset="UTF-8"> <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.
<!DOCTYPE HTML> <html> <head> <meta charset="UTF-8"> <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 (
\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.
<!DOCTYPE HTML> <html> <head> <meta charset="UTF-8"> <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>
<?php //Assign the passed variable to a variable with //a more convenient name. $greeting = $_GET['greet']; ?> <!DOCTYPE HTML> <html> <head> <meta charset="UTF-8"> <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
$_GETarray and can be accessed using the following syntax:
- A shortcut for
echo 'text to print';is
<?= 'text to print' ?>.
Many PHP developers feel that it is best practice to avoid using this shortcut syntax for
echo. One reason for this is that the shortcut syntax makes the resulting PHP file impossible to parse as XML. Another is that the short_open_tag directive must be turned on for it to work. See http://us3.php.net/manual/en/ini.core.php#ini.short-open-tag
Please watch this video on our YouTube channel to get a better understanding of variable scope and superglobal variables.