Chapter 14. Debugging

Common Errors

The most common type of error you'll run across is syntacticalfailing to cross all your t's and dot all your i's, so to speak. Errors like this will result in error messages like the one in Figure 14.1. These are the first types of errors PHP will catch because the code is evaluated for syntax before execution takes place. In order to avoid making these sorts of mistakes when you program, be sure to:

• End every executable line of code with a semi- colon .

• Use a closing quotation mark, parentheses, bracket, or brace for every opening quotation mark, parentheses, bracket , or brace.

• Escape, using the backslash, all single- and double-quotation marks within the print() statement.

One thing you should also understand about syntactical errors is that just because the PHP error message says the error is occurring on line 12, for example, doesn't mean that the mistake is actually on that line. It is not uncommon for there to be a difference between what PHP thinks is line 12 and what your text editor indicates is line 12. So while PHP's direction is useful in tracking down a problem, learn to treat the line number referenced as more of a starting point rather than an absolute.

The second type of error you'll encounter results from trying to do something which cannot be done. These errors occur, for example, when setcookie() or header() is called after the Web browser has already received HTML, when a function is called without the proper arguments, or when you try to write to a file that does not have the proper permissions. These errors are discovered by PHP when attempting to execute the code (Figure 14.2).

The third type of error you'll seeall too frequentlyaren't PHP errors at all, but programmer errors, also called logic errors. (Some programmers would argue that all errors are programmer errors because PHP itself cannot make such mistakes.) One reason such errors happen is because you fail to use the proper variable name. If you do that, you will not see an error message like those in Figures 14.1 and 14.2, you will just witness odd or unpredictable results. These can be the hardest to discern because PHP will not give you a clue as to what the problem is or how to solve it. Nothing but accurate checking and smart detective work will fix them!

For the first exercise, let's add a safety check to the HandleLogin.php script from Chapter 13, Creating Web Applications, to avoid the commonplace occurrence of sending a header after the Web browser has already received some HTML or blank space.

To prevent a common error:

1. Open HandleLogin.php in your text editor (Script 14.1).

   Script 14.1. The original HandleLogin.php page was certainly acceptable but could be improved upon by making use of the headers_sent() function.

   

2. Create a blank line by pressing Return before the initial PHP tag (Script 14.2).

   Script 14.2. The headers_sent() function helps to avoid the common mistake of sending a header (or a cookie) after the Web browser has already received information (Figure 14.3).

   

   This blank line, while it may seem insignificant, will generate the error message seen in Figure 14.3.

   Figure 14.3. Attempting to set a cookie or send a header after the Web browser has already received any information creates an error like this.

   

3. After the initial PHP tag, add the following conditional:

   if ( headers_sent() ) {
        print ("Cannot process your request
       due to a system error!\n");
   } else {
   

   The headers_sent() function returns the value TRUE if any HTML, blank space, etc. has been sent to the Web browser already. If this has occurred, attempting to use the header() function will create a harsh warning in the user 's browser. Instead of having that happen, if the headers have been sent, this page will now show a generic system error.

   If the headers have not been sent, then the headers_sent() function will be false and the rest of the page will be executed as normal.

4. Don't forget to close the headers_sent() conditional before the closing PHP tag!

   }
   

5. Save your script as HandleLogin.php, upload it to the server in the same directory as login.php, and test both pages in your Web browser (Figure 14.4).

   Figure 14.4. By using the headers_sent() function, you can print a less perplexing message to the user. It still doesn't solve the problem, but it is better than having users see messages like that in Figure 14.3.

   

Tip

Some text editors, such as BBEdit for the Macintosh, include utilities to check for balanced parenthesis, brackets, and quotation marks.