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Posted Jan 17, 2002

User Interaction and Forms - Page 3

By Kevin Yank

For many applications of PHP, the ability to interact with users who view the Web page is essential. Veterans of JavaScript tend to think in terms of event handlers, which let you react directly to the actions of the user - for example, the movement of the mouse over a link on the page. Server-side scripting languages such as PHP have a more limited scope when it comes to user interaction. As PHP code is activated when a page is requested from the server, user interaction can only occur in a back-and- forth fashion: the user sends requests to the server, and the server replies with dynamically generated pages.

The key to creating interactivity with PHP is to understand the techniques we can use to send information about a users' interaction along with their request for a new Web page. PHP makes this fairly easy, as we'll now see.

The simplest method we can use to send information along with a page request uses the URL query string. If you've ever seen a URL with a question mark following the filename, you've witnessed this technique in use. Let's look at an easy example. Create a regular HTML file called welcome.html (no .php file extension is required, since there will be no PHP code in this file) and insert this link:

<a href="welcome.php?name=Kevin"> Hi, I'm Kevin! </a>

This is a link to a file called welcome.php, but as well as linking to the file, we're also passing a variable along with the page request. The variable is passed as part of the "query string", which is the portion of the URL that follows the question mark. The variable is called name and its value is Kevin. To restate, we have created a link that loads welcome.php, and informs the PHP code contained in the file that name equals Kevin.

To really understand the results of this, we need to look at welcome.php. Create it as a new HTML file, but this time note the .php extension -- this tells the Web server that it can expect to interpret some PHP code in the file. If your Web server is not configured to accept .php as a file extension for PHP files, you may have to call it welcome.php3 instead (in which case you'll also want to adjust the link above accordingly). In the body of this new file, type:

  echo( "Welcome to our Web site, $name!" );

Now, if you use the link in the first file to load this second file, you'll see that the page says "Welcome to our Web site, Kevin!" The value of the variable passed in the query string of the URL was automatically placed into a PHP variable called $name, which we used to display the value passed as part of a text string.

You can pass more than one value in the query string if you want to. Let's look at a slightly more complex version of the same example. Change the link in the HTML file to read as follows (this is welcome2.html in the code archive):

<a href="welcome.php?firstname=Kevin&lastname=Yank">
Hi, I'm Kevin Yank! </a>

This time, we'll pass two variables: firstname and lastname. The variables are separated in the query string by an ampersand (&). You can pass even more variables by separating each name=value pair from the next with an ampersand.

As before, we can use the two variable values in our welcome.php file (this is welcome2.php in the code archive):

  echo( "Welcome to our Web site, $firstname $lastname!" );

This is all well and good, but we still have yet to achieve our goal of true user interaction, where the user can actually enter arbitrary information and have it processed by PHP. To continue with our example of a personalized welcome message, we'd like to allow the user to actually type his or her name and have it appear in the message. To allow the user to type in a value, we'll need to use an HTML form.

Here's the code (welcome3.html):

<form action="welcome.php" method="get">
First Name: <input type="text" name="firstname" /><br />
Last Name: <input type="text" name="lastname" /><br />
<input type="submit" value="GO" />

Don't be alarmed at the slashes that appear in some of these tags (e.g. <br />). The new XHTML standard for coding Web pages, calls for these in any tag that does not have a closing tag, which includes <input> and <br> tags, among others. Current browsers do not require you to use the slashes, of course, but for the sake of standards-compliance, the HTML code in this series will observe this recommendation. Feel free to leave the slashes out if you prefer (I agree that they're not especially nice to look at).

This form has the exact same effect as the second link we looked at (with firstname=Kevin&lastname=Yank in the query string), except that you can enter whatever names you like. When you click the submit button (which has a label of "GO"), the browser will load welcome.php and automatically add the variables and their values to the query string for you. It retrieves the names of the variables from the name attributes of the input type="text" tags, and it obtains the values from the information the user typed into the text fields.

The method attribute of the form tag is used to tell the browser how to send the variables and their values along with the request. A value of get (as used above) causes them to be passed in the query string, but there is an alternative. It's not always desirable -- or even technically feasible -- to have the values appear in the query string. What if we included a <textarea> tag in the form, to let the user enter a large amount of text? A URL that contained several paragraphs of text in the query string would be ridiculously long, and would exceed by far the maximum length of the URL in today's browsers. The alternative is for the browser to pass the information invisibly, behind the scenes. The code for this looks exactly the same, but where we set the form method to get in the last example, here we set it to post (welcome4.html):

<form action="welcome.php" method="post">
First Name: <input type="text" name="firstname" /><br />
Last Name: <input type="text" name="lastname" /><br />
<input type="submit" value="GO" />

This form is functionally identical to the previous one. The only difference is that the URL of the page that's loaded when the user clicks the "GO" button will not have a query string. On the one hand, this lets you include large values, or sensitive values (like passwords) in the data that's submitted by the form, without their appearing in the query string. On the other hand, if the user bookmarks the page that results from their submission of the form, that bookmark will be useless, as it doesn't contain the submitted values. This, incidentally, is the main reason that search engines like AltaVista use the query string to submit search terms. If you bookmark a search results page on AltaVista, you can use that bookmark to perform the same search again later, because the search terms are contained in the URL.

That covers the basics of using forms to produce rudimentary user interaction with PHP. We'll cover more advanced issues and techniques in later examples.

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