Web Database Applications with PHP and MySQL, 2nd Edition

Common Programming Errors

Now that we've discussed what errors PHP can produce, let's discuss how to fix them. In this section, we focus on situations where error messages aren't produced or are less useful in helping to find the bug. In particular, we focus on the common mistakes that even experienced programmers make.

A Page That Produces Partial or No Output

One of the most common problems in debugging PHP scripts is seeing:

• Nothing rendered by the web browser

• A pop-up dialog box stating that the "Document Contains No Data"

• A partial document when more is expected

Most of these problems are caused not by a bug in a script, but by a bug in the HTML produced by the script or template. For example, if </table>, </form>, or </frame> closing tags are omitted, a document may not be rendered in some browsers.

An HTML problem can often be identified by viewing the HTML page source using the web browser. For example, on a Linux platform running Mozilla or Netscape, the page source can be accessed with a right-mouse click anywhere over the document and by then selecting View Page Source from the pop-up menu.

For compound or hard-to-identify HTML bugs, the W3C validator at http://validator.w3.org/ analyzes the correctness of the HTML and issues a report. It's an excellent assistant for debugging and final compliance checks before delivery of an application. You can enter the URL of your resource into the W3C validator, or you can upload the HTML output and have it checked. The former is easiest, ...