You embed PHP code into a standard HTML page. For example, here's how you can dynamically generate the title of an HTML document:
<html><head><title><?echo $title?></title></head>...
The <?echo $title?> portion of the document is replaced by the contents of the $title PHP variable. echo is a basic language statement that you can use to output data.
There are a few different ways to embed your PHP code. As you just saw, you can put PHP code between <? and ?> tags:
<? echo "Hello World"; ?>
This style is the most common way to embed PHP, but it is a problem if your PHP code needs to co-exist with XML, as XML may use that tagging style itself. If this is the case, turn off this style in the php.ini file with the short_open_tag directive. Another way to embed PHP code is within <?php and ?> tags:
<?php echo "Hello World"; ?>
This style is always available and is recommended when your PHP code needs to be portable to many different systems. Embedding PHP within <script> tags is another style that is always available:
<script language="php" > echo "Hello World";
</script>
One final style, in which the code is between <% and %> tags, is disabled by default:
<% echo "Hello World"; %>
You can turn on this style with the asp_tags directive in your php.ini file. The style is most useful when you are using Microsoft FrontPage or another HTML authoring tool that prefers that tag style for HTML-embedded scripts.
You can embed multiple statements by separating them with semicolons:
<?php
echo "Hello World";
echo "A second statement";
?>
It is legal to switch back and forth between HTML and PHP at any time. For example, if you want to output 100 <br /> tags for some reason, you can do it this way:
<?php for($i=0; $i<100; $i++) { ?><br />
<?php } ?>
Of course, using the str_repeat( ) function here would make more sense.
When you embed PHP code in an HTML file, you need to use the .php file extension for that file, so that your web server knows to send the file to PHP for processing. Or, if you have configured your web server to use a different extension for PHP files, use that extension instead.
When you have PHP code embedded in an HTML page, you can think of that page as a PHP program. The bits and pieces of HTML and PHP combine to provide the functionality of the program. A collection of pages that contain programs can be thought of as a web application.
An important feature of PHP is its ability to include files. These files may contain additional PHP tags. When you are designing a web application, you can break out common components and place them in a single file. This step makes it much easier to change certain aspects in one place later, and have the change take effect across the entire application. To include a file, use the include keyword:
<?php
$title="My Cool Web Application"; include "header.inc";?>
The header.inc file might look as follows:
<html><head><title><?php echo $title?></title></head>
This example illustrates two important concepts of included files in PHP. First, variables set in the including file are automatically available in the included file. Second, each included file starts out in HTML mode. In other words, if you want to include a file that has PHP code in it, you have to embed that code just as you would any other PHP code.
Note also that I used the .inc extension here. This is not a special file type, just an arbitrary extension name I chose. Since your Apache server is not set up to treat .inc files as PHP files, if you put this file somewhere under your document_root, people can browse to it and see the PHP source in that file directly. This is usually not a good idea, so I add these lines to my httpd.conf file:
<Files ~ "\.inc$"> Order allow,deny Deny from all</Files>
This blocks any direct access to .inc files.
The other option is to not put the files under document_root, or perhaps to name them .php instead. But be very careful with that last approach. Keep in mind that people will then be able to execute these scripts, when they were probably not designed to be executed in a standalone fashion.
Other ways to include files are through include_once, require, and require_once. The difference between include and require is simply that with include, if the file to be included does not exist, you get a warning, whereas with require you get a fatal error and script execution stops. The include_once and require_once variations ensure that the file being included has not been included already. This helps avoid things like function redefinition errors.