Professional WEB Development Blog

Environmental Variables

All environmental variables passed to a script are placed by the interpreter into the $HTTP_ENV_VARS array. The format of this array is different on different computers. In Windows, environmental variables can be checked by executing the set command in the command line; in UNIX-like systems, environmental variables can be viewed by executing the env command.

You can find the following PHP environmental variables of use:

• $DOCUMENT_ROOT — The path to the document root directory of the currently executing script on the server.
• $SCRIPT_FILENAME — The current script's path.
• $SERVER_ADDR – The address of the IP server on which the current script is executing.
• $SERVER_PORT — The server port used by the web server for communication.

Error Handling in PHP

At certain configuration settings, PHP may not issue error messages. For production web sites, I recommend to keep this feature disabled. An extra message for a hacker is an extra hint to a successful break-in. For example, a message informing of excessive parameters tells me that the script does not check for the number of parameter passed to it, so it may not do other checks either, for example, whether the system function is called the right way. We will talk about the dangers inherent to this function repeatedly in this book.

A system used for application development must issue messages for any errors; otherwise, it is more likely than not that you will miss some potential errors, and will not be able to understand why the script code is not performing the way you intend it to.

The error reporting feature is enabled by setting the error_reporting parameter in the php.ini file to E_ALL. Error messages may be issued when numerical data is compared with string data. For example, adding a command error_reporting (E_ALL) command at the beginning of the script in which we considered the print_max() function (see Section 2.8) will produce the following error when a number and string are compared:

Arrays in PHP

An array is list of values that can be referenced with a single variable. This is achieved by using an index to reference individual array elements. Either a number or a word can be an index. Number indices start with zero.

Arrays are named in exactly the same way as variables, but with square brackets after the array name. In the following example, words "cake," "bread," and "carrot" are added to an array.

$goods[]= "cake";
$goods[]= "bread";
$goods[]= "carrot";

A particular array element is referenced by specifying its index in square brackets. For example, the following code displays the contents of the zero element, which is "cake":

Jacking Up Voting Results

Voting systems on different sites are constantly developing and programmers are trying to devise protection against visitors jacking up the voting counters. Suppose that you have decided to take a part in a poll conducted by some site and want your preferred answer to prevail. How can this be done? There are many ways. The one to employ depends on the program used to conduct the polling.

Let's consider one vote-boosting method, using the www.download.com site as an example. Here, visitors can vote for their favorite programs. When you see that your favorite program is way down in the ratings, you naturally want to lift it up and help the developers.

In order to know how to pad the votes, you must know how they are counted. The simplest methods use cookie files. These are files in which web servers save any useful for them information. Each web site has its own file, which only it can read. No site can read cookies created by other site servers. When you cast your vote for some cause or issue, the server saves the information about your vote in a cookie file. Let's consider the steps performed when registering votes:

PHP Functions

When I was beginning to learn programming in Pascal, for a long time I could not fathom what functions were needed for. All my programs had flat structure without branchings or any kind. But once I ran into a problem: I had to write a program whose code looked like shown in next code.

print("Select one of the actions<BR>");

print("===========================<BR>");
print("Search <BR>");
print("===========================<BR>");

print("===========================<BR>");
print("Print <BR>");
print("===========================<BR>");

print("===========================<BR>");
print("Exit <BR>");
print("===========================<BR>");

Terminating Programs

Sometimes a situation will arise when a loop execution has to be terminated. Quite often, this is necessary when an error occurs and further execution may have serious consequences. For example, the required file is not available or a user provided wrong parameters. In either case, further script execution may display confidential information or perform some other undesirable actions. Do not experiment in such case and stop the script execution.

Script execution can be interrupted with the exit() function. Script execution is terminated at once when this command is executed. The die() command is an alias for exit(), and both command allow a message to be displayed in the browser to be specified as a parameter. Consider the following classical example of connecting to a database:

PHP Web Hosting

If you are looking for the key to your website design then PHP, PHP Hypertext Processor, just might be the thing. PHP is one of the most popular open source scripting languages and with it you will be able to create beautifully designed dynamic websites. One of its biggest advantages is that it is so very easy to use that even the most inexperienced web developer will be able to use it. Why make life more difficult than it has to be?

PHP and Web Development

As PHP is open source it is also free and to that it will match all of your web design requirements. When looking at PHP Hosting you will quickly notice that it is usually part of a web hosting package known as LAMP for Linux, Apache, MySQL and PHP. Linux is the operating system that is being used, Apache is the web server software, MySQL is the database management system that you will be using and PHP is nothing less than the programming language.

Loops in PHP

Loops are important program flow control. For example, the problem of raising a number to a power that we used as an example when considering the switch statement, can be solved much easier and more efficient using one of the loop statements. A number is raised to a certain power by multiplying it by itself this number of times. For example, the operation of raising 2 to the power of 3 can be written as follows: 2*2*2. But what if a number has to be raised to the power of 100? This task is somewhat more difficult. Even more difficult is the problem when the power is not known in advance. Here is where loops come to the rescue.

The most often used loop is the for loop. It is also the easiest to understand, so we start our study of loops with it. In the general format it looks as the following:

for (start counter value; end counter value; counter step)
 Statements

Let's use the for loop to raise a number to a power. The code for this may look like the following:

Controlling Program Execution

It is a rare program that simply executes from the beginning to the end, because in most cases there are some conditions that can change the program execution flow. Thus, these conditions have to be checked and reacted to in one way or another. Let's consider an example of a site's main page. When a user visits the site for the first time, he or she can be shown some additional information or greeted with some funny presentation to get him interested in the site. For succeeding visits by the same user, the presentation is no longer shown. The script logic for these actions will be something like the following:

• If visiting for the first time, show the presentation before showing the main page.
• Otherwise, show the main page right away.

As another example, we have to do numerous checks to ensure that a script is reliable and secure. For example, if a script is intended to send a mail message, it is a good idea to check whether the address is specified correctly before mailing the message. Here, the logic can be the following:

• If the address format is valid, mail the message.
• Otherwise, don't and issue an error message.

Constants in PHP

Constants are similar to variables in that they are named memory locations holding certain values; unlike variables, however, once a constant was assigned a value at its declaration, it cannot be changed during script execution.

Constants are used to store some frequently used numbers or strings. For example, your site may be programmed for 640-pixel wide pages and you want to switch to using 800-pixel wide pages. If you used number 640 explicitly in your code, you will have to find all instances it was used in the code and change it. Even though this task can be automated, there is no guarantee that you will find all the numbers that need to be changed or not change a number 640 referring to something other than the page width. Instead of using the number 640 explicitly, you can declare a constant, for example $PgWdth, at the beginning of the file, set it to 640, and then use the constant throughout the file wherever you need to use number 640. Then, if you need to change 640 to 800, all you need to do is to reassign the value of the constant $PgWdth to 800 once at the beginning of the file.

I recommend always using constants or at least variables if a number or a string is used more than once in the code. These constants and variables can be stored in a separate file, which can then be included into the PHP files using these constants or variables. Based on my personal experience, I can tell that using constants can make software maintenance and modification significantly easier.