Chapter 19. GENERATING GRAPHICS

Dynamic Buttons..............................................................................................

Generating Graphs on the Fly.....................................................................

Bar Graphs

Pie Charts

Stretching Single-Pixel Images

This chapter explores generating graphics using the GD extension functions described in
Chapter 12. "Image Functions." It is important to be aware of the issues involved with
the creation of graphics on the fly. The first is that it is relatively costly in terms of CPU
time. In most cases the flexibility of dynamic graphics is not worth what you pay in the
load imposed on the server. Another issue is that making nice-looking graphics from PHP
functions is not easy. Common techniques like drop shadows are next to impossible. As
you will see in the examples that follow, a lot of work goes into creating simple, flat
charts. Last, while there is adequate support for text, functions you'd expect in a word
processor do not exist. Text does not wrap at line ends. There is no concept of leading,
spacing, or descenders. Regardless, generating graphics makes sense in some situations.
This chapter contains some real examples that you can start using with very little
modification.

In the first edition of this text, the examples in this chapter created GIF images. Since
then, the GD library has dropped support for GIFs because a key component of the GIF
standard relies on a patented process for compressing data. Instead, the GD library now
produces PNG and JPEG images. Turn back to Chapter 12 for more information on
this issue.

Dynamic Buttons

Images wrapped in anchor tags are a common navigational device. Instead of plain text,
this method allows you to create buttons similar to those created in the operating system,
or even fanciful icons. In most cases it is best to leave these as graphics created in your
favorite graphics editor, because the time between changes is relatively long. On the
other hand, if you have a button that changes often, it may make sense to create it
dynamically with PHP. The content of the button, the label, needs to be available as a
string in PHP. It could be a statement setting the value of a variable. It could also be a
value retrieved from a file or a database.

An illustration will make this idea clear. Many corporate Web sites have a section for
press releases. Instead of just a list of text links, your client wants a graphic of a flaming
newspaper for each press release, all under the title "Hot off the Press." Each burning
newspaper has text over the top with the headline from the press release. With a small