1. Predefined Variables & Superglobals

We already discussed variables, but we did not already discuss the variables that come ready-made in PHP. Predefined variables are already defined, as their name implies, and can be used without you having to create them first.

The majority of predefined variables are used to provide information from and about the web server, the web browser, and the user. They are called superglobals.

The following superglobals are available:

We will discuss many of these superglobals in detail on the following pages, so for now we will only briefly demonstrate how they are used with a demonstration of the $_SERVER superglobal.

Superglobals, such as the $_SERVER variable, are arrays, which we have not yet learned about. The are a single variable that contain multiple values, all of which are separate and can be referenced separately. The brackets and single quotes at the end of the variable contain the reference name of the real value that we want. The result of the above example is:

The $_SERVER variable has over thirty elements available, most of which are actually useful. The most common elements, assuming your server supports them, are:

2. Getting Form Field Data

Obtaining information that is entered into an HTML form is actually a simple process using PHP. All you need to know is the method used to send the form data, the name of each form field, and what you want to do with the information that you obtain.

When the form method used is "get", the information entered into the form is sent via the URL when the form is submitted. It is visible to everyone, and limited to 100 characters.

Using the $_GET superglobal along with the name of the form field, we can obtain the value of each field.

As you can see, the results of these functions can be echoed, stored in a variable, etc.Type In Something:          

Obviously, using the GET method is limiting in both size and security, so use it wisely. It is useful if you want to allow your users to bookmark a page to come back to later, because they will be returned to the page with the same information already submitted.

3. Posting Form Field Data

The method of obtaining information that is entered into an HTML form using the POST method is nearly identical to obtaining information that uses the GET method. The only difference? The spelling!

When the form method used is "post", the information entered into the form is sent behind-the-scenes when the form is submitted. It is not visible in the address bar, but it is limited to 8MB in size. (Don't worry about the limit; that's a LOT of data!).

Using the $_POST superglobal along with the name of the form field, we can obtain the value of each field.

If you tried the demo above you will see that the HTML form, when submitted, uses $_SERVER['PHP_SELF'] to send you right back to the page that the form was submitted from. If you had typed anything into the form, it is echoed directly onto the page.

This method is used for the majority of form-related tasks.

4. Getting & Posting Checkbox & Radio Button Results

Obtaining data from radio buttons and checkboxes can be a little tricky, generally due to a lack of understanding of how radio buttons and checkboxes work.

It is important to remember two facts:

• Checkbox "Names" Must Be Unique to Each Checkbox
• Radio Button "Names" Must Be Identical For Each Group of Buttons

Let's jump right into an example:

The code produces the following result:

Now let's use the $_POST[] superglobal to get the form data that was submitted. It is best to begin by checking to see if the form was submitted. This can be done using the isset() function to see if the submit button has been clicked.

As you can see, the name of each $_POST[] superglobal corresponds with the name of the HTML form field in question.

Next, we can check to see if the submitted form should be graded. Checkboxes only return a value if they are checked. If a checkbox is not checked, then no value will be sent, so we can use the empty() function to determine our next course of action.

Now we can find out which answer was chosen.

And at last, we can determine whether or not it was the right answer.

Submit the form in the example at the top of the page to see the results below. (Notice that the form action will return you to this page when the form is submitted.)

5. PHP File Uploads

Allowing anyone and everyone to upload files to your web server opens up many, many concerns about security, but we will not address them all here. Instead, we will focus on the very basic mechanics of uploading files so that you can experiment with this feature.

To begin, you will need an HTML file upload form with a method of "post" and a specific encoding type, such as the following example. (The action should lead to the php file where the file upload script is located.)

When a file is uploaded, it gets stored in a temporary area on the server until it is moved. The file has to be moved from that area, or else it will be destroyed. In the meantime, the $_FILES[] superglobal array is filled up with data about the uploaded file. Since the file's upload field in our example is called "my-file", the following data is created:

To begin the file upload script, we will use the is_uploaded_file() function as an alternative to the isset() and empty() functions to verify that a file has been uploaded to its temporary location.

The next step, if you want to only accept certain file types and sizes, would be to check out those factors and send out error messages accordingly. In our example we'll move right along and determine whether or not the file has already been uploaded, since we do not want to over-write an existing file. We can use the file_exists() function for this purpose.

And now, at long last, we can use the move_uploaded_file() function to move the temporary file into its permanent location.

That's all it takes!

Two common problems that you may run into, causing the upload process to fail, are the file size and directory permissions. PHP sets a default "upload_max_filesize" to limit the size of the file uploaded. The default is 2M (megabytes) and any file that exceeds this limit will not upload. Also, if the directory (folder) where you try to move the file must have certain permissions set, or you will not be allowed to move the file into that directory.

6. PHP Url Functions

PHP provides several functions used to deal with URL strings. A URL is an address used to identify a website, and a URL string might look something like this: http://www.binitpatel.co.in/Chapter-7.aspx The URL functions that we will learn about here are:

Am I the only one that wants all that translated into English?! Let's begin with get_headers(), which will return an array containing the headers sent by the server in response to an HTTP request.

..ooookkkkaaayyy... moving on (to something more useful?), the get_meta_tags() function returns an array containing the meta tags found between the HTML <head> and </head> tags of the specified webpage.

The parse_url() functions takes a url and returns it to you in little pieces stored in an array.

The urlencode() and rawurlencode() functions both encode strings by replacing all non-alphanumeric characters such as spaces and symbols with placeholders that can later be decoded to return the original string. These functions are ideal for preserving strings used in URL queries, or even passed between pages as "GET" data when a form is submitted.

The urldecode() and rawurldecode() functions can be used to decode the strings once they reach their destination and need to be used or displayed in their original format.

The urlencode() function works with urldecode() and the rawurlencode() function works with rawurldecode(). The difference between the two sets of functions is only in the style that they used to encode each string.

7. Cookies Vs. Sessions

Before we learn how to create cookies and sessions it is important to understand the difference between these two similar (and yet very different) methods of storing information. And before we begin to explain that, let's get one very important detail out of the way; the kind of cookies that we will be discussing are no more edible than my keyboard. (And yes, my keyboard is plastic.)

The basic task of both cookies and sessions is to store visitor data so that it can be accessed by every page on a website. This data is usually provided by the visitor and used to customize the experience, including their visit statistics, identity, setting and/or preferences, etc.

• Cookies are small files that are stored in the visitor's browser.
• Cookies can have a long lifespan, lasting months or even years.
• Cookies are limited in size depending on each browser's default settings.
• Cookies can be disabled if the visitor's browser does not allow them (uncommon).
• Cookies can be edited by the visitor. (Do not use cookies to store sensitive data.)

• Sessions are small files that are stored on the website's server.
• Sessions have a limited lifespan; they expire when the browser is closed.
• Sessions are only limited in size if you limit their size on the server.
• Sessions cannot be disabled by the visitor because they are not stored in the browser.
• Sessions cannot be edited by the visitor.

In short, cookies serve as a temporary or long-term storage unit on the visitor's computer that should not contain sensitive information, and sessions serve as a temporary storage unit not on the visitor's computer that can hide sensitive information. For most tasks I find it efficient to use sessions and cookies together. By giving each visitor a cookie with a unique ID, I can use that cookie to recognize each visitor when they return. I can then use sessions to handle the page-to-page data exchange that actually provides each visitor with their customized settings and information, which are provided by each visitor and stored in a database until they are reference by the unique ID stored in the cookie. Now that's wasn't too hard to swallow, was it? But, now that you know the difference between sessions and cookies, feel free to skip over actually learning them, until you plan to use them... they are not yet vital knowledge.

8. PHP Cookies

Cookies are small files that are stored in the visitor's browser.Cookies can be used to identify return visitors, keep a user logged into a website indefinitely, track the time of the user's last visit, and much more.

Cookies accept seven different arguments, but only the "name" is required. (Keep in mind that all values are stored on the visitor's computer, so the data is not private. Never store passwords in cookies, for example!) The options are:

PHP allows you to create, retrieve and update cookies. The setcookie() function is used to first create a cookie. This function must be run before any other data is sent to the browser, such as the opening <html> tag or random whitespace. The syntax is: setcookie(name, value, expire, path, domain);

Here we have set a cookie called "Example" that has a useless value of "Whatever Value I Want" and an expiration date one month in the future. (We have not yet studied php date and time functions, so I will briefly clarify that the time() function collects the current timestamp and adds 2591000 seconds to it, which is 30 day's worth of seconds, thus creating an expiration date 30 days in the future.)

Now, we are free to retrieve the "value" that is stored in our cookie, using the $_COOKIE superglobal. It is best to use the isset() function for this, because if a cookie has not been set on the computer that we are trying to retrieve it from, headaches can result.

Here we check to see if the cookie that we created really was created, and echo the value if it was. The value that will be echoed is "Whatever Value I Want". A more practical example might have stored the user's name or preferences, but it's fun to be unpredictable.

Deleting a cookie is as simple as using the setcookie() function with an expiration date in the past. Since there are 3600 seconds in an hour, using -3600 will set the expiration date an hour previous and the cookie will expire and disappear.

Finally! A cookie that your mother won't limit your intake of!

9. PHP Sessions

Sessions are small, temporary files that are stored on the website's server. Sessions can be used to store unique information about, or change unique settings for each visitor, but only for the duration of each separate visit.

Sessions work by assigning each visitor a unique string that is used to identify them as they move from page to page. This prevents one visitor's information from being mixed up with another's.

PHP allows you to create and destroy a session, as well as assign, retrieve and update session variables. The session_start() function is used to first create a session, or to continue the current session if one already exists. This function must be run on every page, before any other data is sent to the browser, such as the opening <html> tag or random whitespace. The code is very simple.

After starting the session you can store session variables in the $_SESSION superglobal, as well as update and retrieve them. Each value stored in this array is assigned a "key" or name by which to identify the specific value. The syntax is: $_SESSION['key'] = value;

Due to the nature of sessions, you can assign a session value on one page and retrieve it on a completely separate page, as long as session_start() is set at the beginning of the file. The best practice is to check and see if each session value exists before retrieving it, by using the isset() function.

Although sessions will end themselves and erase all session variables after a period of inactivity, it is possible to speed up the process. This is necessary if you are using sessions to keep a person logged into a website, for example, and they click "logout". The unset() function will free (erase) a specific session variable, and the session_destroy() function will end the session completely, as well as erase all session variables.