PHP and MySQL Web Development: Ajax Basics
This chapter is from the book
This chapter is from the book
This chapter is from the book
The World Wide Web began as a series of static pages containing text and links to image, audio, and video files. For the most part, the Web still exists in this state, although many of these pages filled with text and multimedia are dynamically generated through server-side scripting; this is what you have created through the applications in this book. But the advent of Web 2.0 has led developers to attempt to find new methods of user interaction with the web servers and databases that store the information we desire. One increasingly popular method of interaction is through the use of Ajax (Asynchronous JavaScript and XML) programming to enhance interactivity while reducing the time spent retrieving static elements.
In this chapter, we introduce the basics of Ajax programming and provide some sample Ajax elements you can integrate into your applications. This chapter is in no way comprehensive, but it will provide a solid foundation for future work with these technologies. Key topics covered include
- The combination of scripting and markup languages used to create Ajax applications.
- The fundamental parts of an Ajax application, which include issuing a request and interpreting a response from the server.
- How to modify elements of applications from previous chapters to create Ajax-enabled pages.
- The availability of code libraries and where to find more information.
What Is Ajax?
Ajax itself is not a programming language or even a single technology. Instead, Ajax programming typically combines client-side JavaScript programming with XML-formatted data transfers and server-side programming via languages such as PHP. Additionally, XHTML and CSS are used for presentation of Ajax-enabled elements.
The result of Ajax programming is typically a cleaner and faster user interface to an interactive application—think of the interfaces to Facebook, Flickr, and other sorts of social networking sites that are at the forefront of Web 2.0. These applications enable the user to perform many tasks without reloading or redrawing entire pages, and this is where Ajax comes into play. Client-side programming invokes a bit of server-side programming, but only in a specific area displayed in the user’s browser, which is then the only area to be redrawn. This action mimics the result of actions in standalone applications, but in a web environment.
A common example is that of working in a spreadsheet application (offline) versus viewing a table full of information on a website. In the offline application, the user could make changes in one cell and have formulas applied to other cells, or the user could sort the data in one column, all without leaving the original interface. In a static web environment, clicking a link to sort a column would require a new request to the server, a new result sent to the browser, and for the page to be redrawn to the user. In an Ajax-enabled web environment, that table could be sorted based on the user’s request, but without reloading the entire page.
In the next few sections, we look at the various technologies that come into play when using Ajax. This information is by no means comprehensive; I provide additional resources throughout.
HTTP Requests and Responses
Hypertext Transfer Protocol, or HTTP, is an Internet standard defining the way web servers and web browsers communicate with each other. When a user requests a web page by typing a URL into the location bar of a web browser, or by following a link, submitting a form, or performing any other task that takes the user to a new destination, the browser makes an HTTP request.
This request is sent to a web server, which returns one of many possible responses. To get an understandable response from the web server, the request has to be properly formed. Knowing the proper formation of requests and responses is critical when using Ajax, because it is the responsibility of the developer to write HTTP requests and expect certain results within the Ajax application.
When making an HTTP request, the client sends information in the following format:
- The opening line, which contains the method, the path to the resource, and the HTTP version in use, such as the following:
GET http://server/phpmysql4e/chapter34/test.html HTTP/1.1
- Other common methods include POST and HEAD.
- Optional header lines, in the format parameter: value, such as:
User-agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1
- and/or
Accept: text/plain, text/html
- For a list of HTTP headers, see http://www.w3.org/Protocols/rfc2616/.
- A blank line
- An optional message body
After making an HTTP request, the client should receive an HTTP response.
The format of an HTTP response is as follows:
- The opening line, or status line, which contains the HTTP version in use and a response code, such as:
HTTP/1.1 200 OK
- The first digit of the status code (in this case the 2 in 200) offers a clue to the response. Status codes beginning with 1 are informational, 2 represents success, 3 represents redirection, 4 represents a client error such as 404 for a missing item, and 5 represents a server error such as 500 for a malformed script. For a list of HTTP status codes, see http://www.w3.org/Protocols/rfc2616/.
- Optional header lines, in the format parameter: value, such as:
Server: Apache/2.2.9 Last-Modified: Fri, 1 Aug 2008 15:34:59 GMT
DHTML and XHTML
Dynamic HTML, or DHTML, is the term used for the combination of static HTML, Cascading Style Sheets (CSS), and JavaScript to work with the Document Object Model (DOM) to alter the appearance of seemingly static web page after all elements have been loaded. At first glance this functionality seems quite similar to an Ajax-enabled site, and in some ways it is. The difference lies in the asynchronous connectivity between the client and server—the “A” in Ajax.
Although a DHTML-driven site may show dynamic movement in navigational drop-downs or in form elements that change depending on the selections previously made, all the data for these elements have already been retrieved. For instance, if you have designed a DHTML site that that shows Section 1 of some text when the user rolls over a link or button, and shows Section 2 of some text when the user rolls over yet another link or button, the text for both Section 1 and Section 2 will already have been loaded by the browser. The developer will have used a bit of JavaScript that sets the CSS attribute for visibility to visible or not, depending on the actions of the user’s mouse. In an Ajax-enabled site, it is likely that the area reserved for Section 1 or Section 2 text will be filled based on the result of a remote scripting call to the server while the rest of the site remains static.
The Extensible Hypertext Markup Language, or XHTML, functions similarly to HTML and DHTML in that all three are used to mark up content for display via a client device (web browser, phone, other handheld device) and allow for the integration of CSS for additional control of the presentation. The differences between XHTML and HTML include the manner in which XHTML conforms to XML syntax and the manner in which XHTML can be interpreted by XML tools in addition to the standard web-browsing tools.
XHTML is written entirely in lowercase for elements (for example, <head></head> instead of <HEAD></HEAD>) and attributes (for example, href instead of HREF). Additionally, all attribute values must be enclosed in either single or double quotation marks, and all elements must be explicitly closed—either by the end tag in a tag pair or in singleton elements such as the <img /> tag or <br/> tag.
For more information on XHTML, see http://www.w3.org/TR/xhtml1/.
Cascading Style Sheets (CSS)
Cascading Style Sheets (CSS) are used to further refine the display of static, dynamic, and Ajax-enabled pages. Using CSS allows the developer to change the definition of a tag, class, or ID within one document (the style sheet) and have the changes take effect immediately in all pages that link to that style sheet. These definitions, or rules, follow a specific format using selectors, declarations, and values.
- Selectors are the names of HTML tags, such as body or h1 (heading level 1).
- Declarations are the style sheet properties themselves, such as background or font-size.
- Values are given to declarations, such as white or 12pt.
Thus, the following is a style sheet entry that defines the body of a document as white, and all text in the document as a normal weight, 12 point, Verdana, or sans-serif font:
body {
background: white [or #fff or #ffffff];
font-family: Verdana, sans-serif;
font-size: 12pt;
font-weight: normal;
}
These values will be in effect for the page until an element is rendered that has its own style defined in the style sheet. For instance, when an h1 is encountered, the client will display the h1 text however it has been defined—probably with a font size greater than 12pt and with a font-weight value of bold.
In addition to defining selectors, you can also define your own classes and IDs within a style sheet. Using classes (which can be used on multiple elements in a page) or IDs (which can be used only once within a page), you can further refine the display and functionality of elements displayed within your website. This refinement is especially important in Ajax-enabled sites because you use predefined areas of your document to display new information retrieved from the remote scripting action.
Classes are defined similarly to selectors—curly braces around the definitions, definitions separated by semicolons. Following is the definition of a class called ajaxarea:
.ajaxarea {
width: 400px;
height: 400px;
background: #fff;
border: 1px solid #000;
}
In this instance, the ajaxarea class, when applied to a div container, produces a 400-pixel wide by 400-pixel high square with a white background and a thin black border. The usage is as follows:
<div class="ajaxarea">some text</div>
The most common method of using style sheets is to create a separate file with all the style definitions in it, and then link to it in the head element of your HTML document, like so:
<head> <link rel="stylesheet" href="the_style_sheet.css" type="text/css"> </head>
For more information on CSS, see http://www.w3.org/TR/CSS2/.
Client-Side Programming
Client-side programming occurs within your web browser after a page has been entirely retrieved from a web server. All the programming functions are included in the data retrieved from the web server and are waiting to be acted upon. Common actions performed on the client side include showing or hiding sections of text or images, changing the color, size, or location of text or images, performing calculations, and validating user input in a form before sending the form to be processed on the server side.
The most common client-side scripting language is JavaScript—the “J” in Ajax. VBScript is another example of a client-side scripting language, although it is Microsoft-specific and thus not a good choice for an open environment in which all manner of operating systems and web browsers may be in use.
Server-Side Programming
Server-side programming includes all scripts that reside on a web server and are interpreted or compiled before sending a response to the client. Server-side programming typically includes server-side connections to databases; requests and responses to and from a database are thus part of the scripts themselves.
These scripts could be written in any server-side language, such as Perl, JSP, ASP, or PHP—the latter being the language used throughout the examples in this chapter for obvious reasons. Because the response of a server-side script is typically to display data marked up in some variant of standard HTML, the end-user environment is of little concern.
XML and XSLT
You were introduced to XML in Chapter 33, “Connecting to Web Services with XML and SOAP,” which included basic information on the format, structure, and use of XML. In the context of Ajax applications, XML—the “X” in Ajax—is used to exchange data; XSLT is used to manipulate the data. The data itself is either sent through or retrieved from the Ajax application you create.
For more information on XML, see http://www.w3.org/XML/, and for more information on XSL, see http://www.w3.org/TR/xslt20/.