A First Script
Hour 3: A First Script
What You'll Learn in This Hour:
How to create, upload, and run a PHP script
How to incorporate HTML and PHP in the same document
How to make your code clearer with comments
You installed and configured PHP in the last hour. It is now time to put it to the test. In this hour, you will create your first script and spend a little time analyzing its syntax. By the end of the hour, you should be ready to create documents that include both HTML and PHP.
Our First Script
Let's jump right in with a PHP script. To begin, open your favorite text editor. Like HTML documents, PHP files are made up of plain text, so you can create them with any text editor, such as Notepad and HomeSite on Windows, Simple Text and BBEdit on Mac OS, or VI and Emacs on Unix operating systems. Most popular HTML editors provide at least some support for PHP.
Keith Edmunds maintains a handy list of PHP-friendly editors at http://phpeditors.linuxbackup.co.uk.
Type in the example in Listing 3.1 and save the file. We'll name our file listing3.1.php.
Listing 3.1 A First PHP Script
1: <?php 2: phpinfo(); 3: ?>
The code in Listing 3.1 causes information about our PHP installation to be output to the browser. The phpinfo() function is very useful for debugging scripts because of the contextual information it provides.
The extension to the PHP document is important because it tells the server to treat the file as PHP code and invoke the PHP engine. The default PHP extension for a PHP document is .php. This can be changed, however, by altering the server's configuration. You saw how to do this in Hour 2, "Installing PHP." System administrators occasionally configure servers to work with non-default extensions, so some server setups might expect extensions such as .phtml or .php5. As you saw in the last hour, for example, Apache uses the AddType directive to determine how a file should be treated. AddType is usually found in Apache's configuration file, httpd.conf:
AddType application/x-httpd-php .php
If you are not working directly on the machine that will be serving your PHP script, you will probably need to use an FTP client, such as WS_FTP for Windows or RBrowser Lite for MacOS, to upload your saved document to the server.
For historical reasons, different operating systems use different character combinations to denote the end of a line of text. You should save your PHP documents with the correct line breaks for the operating system that runs your server. A document with the wrong line breaks for the operating system might be read as a single very long line of text by the PHP engine. This usually causes no problems, but the occasional bug can result. Most good text editors allow you to nominate your target operating system.
After the document is in place, you should be able to access it via your browser. If all has gone well, you should see the script's output. Figure 3.1 shows the output from the listing3.1.php script.
If PHP is not installed on your server or your file's extension is not recognized, you might not see the output shown in Figure 3.1. In these cases, you might see the source code created in Listing 3.1, or you might be prompted to download the file! The effect of a misconfiguration depends your platform, server and browser. Figure 3.2 shows what happens to Internet Explorer when an unknown extension is encountered by Apache running PHP as a module on Linux.
Figure 3.1 Success: The output from Listing 3.1.
Figure 3.2 Failure: The extension is not recognized.
If this happens, or you see the script's source code in the browser window, first check the extension with which you saved your PHP script. If you are used to working with HTML files, for example, check that you have not saved your script with a .html extension. If the file extension is as it should be, you might need to check that PHP has been installed properly and that your server is configured to work with the extension you have used for your script. You can read more about installing and configuring PHP in Hour 2. To produce the output shown in Figure 3.2, we removed the AddType directive from Apache's configuration file.
Configuration problems vary between servers and platforms. Omitting the Action directive from Apache's httpd.conf file when running PHP as a CGI results in errors, for example.
If you encounter unexpected behavior, you should refer to the installation instructions for your server. You can find complete instructions for setting up PHP with most servers at http://www.php.net/installation.
Now that you have uploaded and tested your script, you can take a look at the code in a little more detail.
Beginning and Ending a Block of PHP Statements
When writing PHP, you need to inform the PHP engine that you want it to execute your commands. If you don't do this, the code you write will be mistaken for HTML and will be output to the browser. You can do this with special tags that mark the beginning and end of PHP code blocks. Table 3.1 lists four such PHP delimiter tags.
Table 3.1 PHP Start and End Tags
Of the tags in Table 3.1, only the standard and the script tags can be guaranteed to work on any configuration. The short and ASP style tags must be explicitly enabled in your php.ini, which you examined in Hour 2.
To activate recognition for short tags, you must make sure that the short_open_tag switch is set to "On" in php.ini, like so:
short_open_tag = On;
Short tags are enabled by default, so you need to edit php.ini only if you want to disable these.
To activate recognition for the ASP tags, you must enable the asp_tags setting, like so:
asp_tags = On;
After you have edited php.ini, you should be able to choose from any of the four styles for use in your scripts. Having said this, it is not advisable to use anything but the standard <?php ?> tags. It is the officially supported syntax, it works well with XML, and it works in any PHP context. Furthermore, there is no guarantee that short tags will be supported forever.
The character sequence <? tells an XML parser to expect a processing instruction:
<?xml version="1.0" encoding="UTF-8"?>
This syntax clashes with the PHP short tag. Used as part of an XML or XHTML document, PHP short tags could confuse an XML validator, the PHP engine, or both. Disable short tags if you intend to use PHP in an XML context.
You should also avoid short or ASP tags if you want your script to be portable. Third-party servers might not be configured to support these tags.
Let's run through some of the ways in which you can legally write the code in Listing 3.1. You could use any of the four PHP start and end tags that you have seen:
<? phpinfo(); ?> <?php phpinfo(); ?> <% phpinfo(); %> <script language="php"> phpinfo(); </script>
Single lines of code in PHP also can be presented on the same line as the PHP start and end tags, like so:
<?php phpinfo(); ?>
Now that we have proved to ourselves that PHP is working, let's move on and write some more code.
The print() Function
In Listing 3.1 we used the phpinfo() function. In Listing 3.2 we change our script so that we print something to the browser for ourselves.
Listing 3.2 Printing a Message
1: <?php 2: print "hello world"; 3: ?>
print() is a language construct that outputs data. Although it is not a function, it behaves like one: It accepts a collection of characters, known as a string. Strings must be enclosed by quotation marks, either single or double. The string passed to print() is then output, usually to the browser or command line.
Similar to the print() statement is echo(), which behaves in the same way (except that it does not return a value). For most examples in this book, you could replace all uses of print() with echo() without any noticeable effect.
Function calls generally require parentheses after their names, regardless of whether they demand arguments. print() is, strictly speaking, a language construct rather than a function and does not demand the use of parentheses. As parentheses are omitted by convention, we will usually omit them in our examples.
We ended our only line of code in Listing 3.2 with a semicolon. The semicolon informs the PHP engine that we have completed a statement.
A statement represents an instruction to the PHP engine. Broadly, it is to PHP what a sentence is to written or spoken English. A sentence should end with a period; a statement should usually end with a semicolon. Exceptions to this include statements that enclose other statements and statements that end a block of code. In most cases, however, failure to end a statement with a semicolon confuses the PHP engine and results in an error being reported at the following line in the script.
Because the statement in Listing 3.3 is the final one in that block of code, the semicolon is optional.