Beginning PHP6, Apache, MySQL Web Development- P1 docx

9 Handling and Avoiding Errors You will probably be spending a fair amount of time contemplating errors in your code, as do most web developers. No matter how good you are, how well you code, how long you have been coding, or how hard you try, you will encounter times when you have errors in your code. It is of the utmost importance that you know how to handle your errors and debug your own code. Being able to efficiently and properly debug your code is an invaluable time saver, and in web development, $time == $money! Luckily, PHP provides you with many ways to isolate and resolve most, if not all, of these unwanted errors. PHP also allows you to capture the errors and create your own custom error functions or pages. These features are useful when debugging your code and when notifying your webmaster about errors that seem to be happening to your applications as users are running them. Not only can you use PHP code to trap errors and customize the error messages, you can use the Apache web server to help do this. How the Apache Web Server Deals with Errors Apache has a directive, the ErrorDocument , that you can configure in the httpd.conf file to create custom error pages with PHP, so visitors to your site do not see the default server - based error pages, which may not be as helpful or descriptive as customized error messages. You have limitless possibilities when creating these custom messages. As with the PHP error - catching pages, you can have the ErrorDocument call PHP pages to do whatever you would like them to do — from simply displaying a friendly error message to the user, to e - mailing a system administrator to notify him or her of the failure. c09.indd 241c09.indd 241 12/10/08 5:59:28 PM12/10/08 5:59:28 PM Part I: Movie Review Web Site 242 Unlike PHP error pages, the Apache ErrorDocument pages are used more for instances of missing pages (that is, a Page Not Found error or Forbidden Access error pages and other requests of that sort). So, if someone visits your site and runs into the Page Not Found error page, the script will e - mail the administrator, who can in turn check to see whether this was a valid request and there is something wrong with the page or server, or whether someone was just looking for the wrong pages, or if this was a malicious user trying to sniff around where he or she wasn ’ t supposed to be. Apache ’ s ErrorDocument Directive Error handling is an invaluable resource and a must have for web developers, to keep their sites up and running with the fewest end - user problems or complaints. If you rely solely on people contacting you to tell you about errors on your site, it is difficult to have a smoothly running server. Allowing the server to do this for you will greatly increase your success at running a smooth server. This section first looks at Apache ’ s ErrorDocument method of error handling. Try It Out Using Apache ’ s ErrorDocument Method First of all, you need to make some changes to the httpd.conf file to allow you to create a custom error page. Apache is usually set up by default to go to its own internal error pages, but you don ’ t want that. You want Apache to go to your custom error page, no matter what error has occurred. To do this, you change the default settings to your own specific settings by following these steps: 1. Open up your httpd.conf file, and you will find some lines that look like this: # # Customizable error responses come in three flavors: # 1) plain text 2) local redirects 3) external redirects # # Some examples: #ErrorDocument 500 “The server made a boo boo.” #ErrorDocument 404 /missing.html #ErrorDocument 404 “/cgi-bin/missing_handler.pl” #ErrorDocument 402 http://www.example.com/subscription_info.html # 2. Change that information to the following, and then restart Apache: # # Customizable error responses come in three flavors: # 1) plain text 2) local redirects 3) external redirects # # Some examples: ErrorDocument 400 /error.php?400 ErrorDocument 401 /error.php?401 ErrorDocument 403 /error.php?403 ErrorDocument 404 /error.php?404 ErrorDocument 500 /error.php?500 c09.indd 242c09.indd 242 12/10/08 5:59:28 PM12/10/08 5:59:28 PM Chapter 9: Handling and Avoiding Errors 243 How It Works You have just edited Apache ’ s configuration file to help you with error handling. By using the ErrorDocument directive, you are able to send users to specific error pages, depending on what error the server has encountered. For example, if you receive a 404 error, the typical “ Page Cannot Be Found ” page, you can redirect it to a page you have created to look like your web site, while still getting the message through to the user that there has been a problem. You can do that with any and all error messages that the server can encounter. Many ErrorDocument codes exist, but we will focus on the error messages you see typically in everyday web browsing: 400: Bad Request 401: Authorization Required 403: Forbidden 404: Not Found 500: Internal Server Error Numerous other error codes exist, of course. You can find a complete list at http://rfc.net/ rfc2616.html#p57 . Although you are seeing just a few error codes in this exercise, you can catch others as well by simply adding another ErrorDocument to the httpd.conf file. For example, if you want to implement the 501 error code, you simply add ErrorDocument 501 /error.php?501 to your code and add the error handling in the error.php page, which you ’ ll see shortly. Next, you ’ ll see a simple way to show the user error messages, and then get into some more complex ways to notify the webmaster of errors occurring on the web site by using the mail() function, which you learned previously. Try It Out Displaying Custom Error Messages To show the user error messages, follow these steps: 1. Open your text editor, and save a page called error.php . 2. Enter the following code: < html > < head > < title > Beginning PHP6, Apache, MySQL Web Development Custom Error Page < /title > < /head > < body > < ?php switch ($_SERVER[‘QUERY_STRING’]) { case 400: ❑ ❑ ❑ ❑ ❑ c09.indd 243c09.indd 243 12/10/08 5:59:29 PM12/10/08 5:59:29 PM Part I: Movie Review Web Site 244 echo ‘ < h1 > Bad Request < /h1 > ’; echo ‘ < h2 > Error Code 400 < /h2 > ’; echo ‘ < p > The browser has made a Bad Request. < /p > ’; break; case 401: echo ‘ < h1 > Authorization Required < /h1 > ’; echo ‘ < h2 > Error Code 401 < /h2 > ’; echo ‘ < p > You have supplied the wrong information to access a secure ‘ . ‘resource. < /p > ’; break; case 403: echo ‘ < h1 > Access Forbidden < /h1 > ’; echo ‘ < h2 > Error Code 403 < /h2 > ’; echo ‘ < p > You have been denied access to this resource. < /p > ’; break; case 404: echo ‘ < h1 > Page Not Found < /h1 > ’; echo ‘ < h2 > Error Code 404 < /h2 > ’; echo ‘ < p > The page you are looking for cannot be found. < /p > ’; break; case 500: echo ‘ < h1 > Internal Server Error < /h1 > ’; echo ‘ < h2 > Error Code 500 < /h2 > ’; echo ‘ < p > The server has encountered an internal error. < /p > ’; break; default: echo ‘ < h1 > Error Page < /h1 > ’; echo ‘ < p > This is a custom error page < /p > ’; } echo ‘ < p > < a href=”mailto:sysadmin@example.com” > Contact < /a > the system ‘ . ‘administrator if you feel this to be in error. < /p > ’; ? > < /body > < /html > 3. Open your browser and type http://localhost/nonexistent/page.html , or any other page you know for certain doesn ’ t reside on your server, into the address bar. You should see the Page Not Found message on the screen, similar to the message shown in Figure 9 - 1 . c09.indd 244c09.indd 244 12/10/08 5:59:29 PM12/10/08 5:59:29 PM Chapter 9: Handling and Avoiding Errors 245 4. Another way to test or simulate the error messages, so that you can ensure you coded the page correctly, is to supply the page with the query string information via the browser. For example, to simulate an Internal Server Error error message, type http://localhost/ error.php?500 into your address bar. The page will use the query string information and run the code just as if there were an Internal Server Error on one of your pages. The result will look pretty similar to the previous example, but will contain a different message. The Internal Server Error page will look like the one shown in Figure 9 - 2 , displaying the Internal Server Error message on the screen. Figure 9-1 c09.indd 245c09.indd 245 12/10/08 5:59:29 PM12/10/08 5:59:29 PM Part I: Movie Review Web Site 246 How It Works You have just created a simple error - handling PHP page. You created a PHP page that will handle the most common errors that servers encounter. By using the query string information along with the switch() statement, you are able to display custom error message pertinent to the error itself. This is useful if you don ’ t want Apache to display its somewhat cryptic - looking error message to your users. Apache ’ s ErrorDocument: Advanced Custom Error Page Up until this point, you ’ ve been showing the user a custom error message only. You can do countless other things, such as e - mailing the administrator or webmaster of the site, so he or she can look into the issue further should there be a problem with certain pages. This is a great way for you to keep track of your pages without having to check up on the server periodically. More than likely, if you haven ’ t received any error e - mails, there haven ’ t been problems with your server. Figure 9-2 c09.indd 246c09.indd 246 12/10/08 5:59:30 PM12/10/08 5:59:30 PM Chapter 9: Handling and Avoiding Errors 247 Try It Out Creating an Error E - Mail In this exercise, you will create a script that generates an automatic e - mail that tells the administrator what time the error occurred, on what day, what the error was, what page generated the error, and what error message was displayed to the user who navigated to the page. 1. Open your error.php file, and add to it the code highlighted here: < html > < head > < title > Beginning PHP6, Apache, MySQL Web Development Custom Error Page < /title > < /head > < body > < ?php switch ($_SERVER[‘QUERY_STRING’]) { case 400: echo ‘ < h1 > Bad Request < /h1 > ’; echo ‘ < h2 > Error Code 400 < /h2 > ’; echo ‘ < p > The browser has made a Bad Request. < /p > ’; break; case 401: echo ‘ < h1 > Authorization Required < /h1 > ’; echo ‘ < h2 > Error Code 401 < /h2 > ’; echo ‘ < p > You have supplied the wrong information to access a secure ‘ . ‘resource. < /p > ’; break; case 403: echo ‘ < h1 > Access Forbidden < /h1 > ’; echo ‘ < h2 > Error Code 403 < /h2 > ’; echo ‘ < p > You have been denied access to this resource. < /p > ’; break; case 404: echo ‘ < h1 > Page Not Found < /h1 > ’; echo ‘ < h2 > Error Code 404 < /h2 > ’; echo ‘ < p > The page you are looking for cannot be found. < /p > ’; break; case 500: echo ‘ < h1 > Internal Server Error < /h1 > ’; echo ‘ < h2 > Error Code 500 < /h2 > ’; echo ‘ < p > The server has encountered an internal error. < /p > ’; break; default: echo ‘ < h1 > Error Page < /h1 > ’; echo ‘ < p > This is a custom error page < /p > ’; } c09.indd 247c09.indd 247 12/10/08 5:59:30 PM12/10/08 5:59:30 PM Part I: Movie Review Web Site 248 echo ‘ < p > < a href=”mailto:sysadmin@example.com” > Contact < /a > the system ‘ . ‘administrator if you feel this to be in error. < /p > ’; $now = (isset($_SERVER[‘REQUEST_TIME’])) ? $_SERVER[‘REQUEST_TIME’] : time(); $page = (isset($_SERVER[‘REQUEST_URI’])) ? $_SERVER[‘REQUEST_URI’] : ‘unknown’; $msg = wordwrap(‘A ‘ . $_SERVER[‘QUERY_STRING’] . ‘ error was encountered on ‘ . date(‘F d, Y’, $now) . ‘ at ‘ . date(‘H:i:sa T’, $now) . ‘ when a ‘ . ‘visitor attempted to view ‘ . $page . ‘.’); mail(‘admin@example.com’, ‘Error from Website’, $msg); ? > < /body > < /html > How It Works The output that you see in the browser will be the same as you saw before, but behind the scenes, the mail() function is used to send an e - mail to the administrator. The mail() function allows you to e - mail anyone you desire when an error occurs. You will learn about the mail() function in more detail in Chapter 11. That ’ s it! You just used Apache ’ s ErrorDocument directive to help you maintain your server. Error Handling and Creating Error - Handling Pages with PHP This section looks at how you can troubleshoot your PHP scripts using simple, logical steps. But first, you need to understand what PHP does when it encounters an error and what it does with certain errors. When a PHP script gets executed and encounters an error, it displays a message in the browser showing you what the error was. Depending on what type of error occurred, the script may not finish executing. You are likely to run into these sorts of errors when writing your own scripts. Don ’ t feel ashamed if you receive errors; everybody makes errors when writing code, no matter what their level of expertise. Even though it is normal to receive errors during the development of your script, you don ’ t want those errors (which are usually too complicated for the layperson to understand) popping up to end users, when your site has gone live. For this reason, it ’ s important to know how to catch those unwanted errors and generate more user - friendly errors that let the user know that there will be a solution forthcoming. c09.indd 248c09.indd 248 12/10/08 5:59:30 PM12/10/08 5:59:30 PM Chapter 9: Handling and Avoiding Errors 249 Error Types in PHP There are 13 predefined error constants that correspond to different types of errors in PHP. They are listed below, along with the E_ALL option. Each of these can be called by either an integer value or a named constant, but because the integer value they represent may change between different versions of PHP (as the value of E_ALL did in PHP 5.2), we recommend only using the constant name. E_ERROR : Fatal runtime errors that cannot be recovered from; the execution of the script is halted . E_WARNING: Nonfatal runtime errors . E_PARSE: Compile - time parse errors . E_NOTICE : Nonfatal runtime notices that indicate that the script encountered something that might be an error, but could also happen in the normal course of running a script . E_CORE_ERROR: Fatal errors that occur during PHP ’ s initial startup; the execution of the script is halted . E_CORE_WARNING : Nonfatal errors that occur during PHP ’ s initial startup . E_COMPILE_ERROR : Fatal compile - time errors; the execution of the script is halted . E_COMPILE_WARNING : Nonfatal compile - time errors . E_USER_ERROR : User - generated error messages (like E_ERROR , but instead generated by using the trigger_error() function); the execution of the script is halted . E_USER_WARNING: User - generated warning messages (like E_WARNING , but instead generated by using the trigger_error() function) . E_USER_NOTICE : User - generated notice messages (like E_NOTICE , but instead generated by using the trigger_error() function) . E_STRICT : Runtime notices that suggest changes to your code that would ensure the best interoperability and forward compatibility of your code . E_RECOVERABLE_ERROR: Catchable fatal errors that indicate that a probably dangerous error occurred, but did not leave the PHP ’ s execution engine in an unstable state . E_ALL : All errors and warnings combined . Before version 6 of PHP, E_ALL combined all errors and warnings except for E_STRICT . Typically, you don ’ t have to worry about all of the error types; your main concern is with runtime errors such as notices, warnings, and errors, along with the user - generated equivalents. The simple, more trivial errors, such as warnings, aren ’ t useful to users but can be helpful to you, since they notify you that you forgot to initialize a variable or something similar. Because initializing variables is purely for your benefit while you are coding to track down errors before your web site launch, it is of no use to display these errors to users once your site goes live. Your error - handling code helps resolve these cryptic errors, to offer helpful, user - friendly messages. ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ ❑ c09.indd 249c09.indd 249 12/10/08 5:59:30 PM12/10/08 5:59:30 PM Part I: Movie Review Web Site 250 The three main types of errors discussed here are: Fatal errors: Fatal runtime errors. These indicate errors that the program can ’ t recover from. Script execution is halted. Warnings: Runtime warnings (nonfatal errors). Script execution is not halted. Notices: Runtime notices. These indicate that the script has encountered something that could indicate an error, but that could also happen in the normal course of running the script. Generating PHP Errors Now let ’ s generate some errors so that you can check out what you need to do to resolve them. Consider this code snippet, for example: < ?php //set string with “Wrox” spelled wrong $string_variable = ‘Worx books are awesome!’; //try to use str_replace to replace Worx with Wrox //this will generate an E_WARNING //because of wrong parameter count str_replace(‘Worx’, ‘Wrox’); ? > If you run this snippet, you should see the following error: Warning: Wrong parameter count for str_replace() in C:\Program Files\Apache Software Foundation\Apache2.2\htdocs\warning_test.php on line 8. The error occurred because str_replace() requires a third parameter for the function. The third parameter is the variable, $string_variable , or a string of text in which you want to search for the first parameter, “ Worx, ” and replace it with “ Wrox. ” Because this is a nonfatal error that does not halt script execution, you can still run code after the point where the error occurred. If you change the snippet to this: < ?php //set string with “Wrox” spelled wrong $string_variable = ‘Worx books are awesome!’; //try to use str_replace to replace Worx with Wrox //this will generate an E_WARNING //because of wrong parameter count str_replace(‘Worx’, ‘Wrox’); //this is a non-fatal error, so the original //variable should still show up after the warning echo $string_variable; ? > ❑ ❑ ❑ c09.indd 250c09.indd 250 12/10/08 5:59:31 PM12/10/08 5:59:31 PM [...]... everything but building that dream site c10.indd 263 12/10/08 5:59:49 PM Part II: Comic Book Fan Site So we will assume you have a great idea for a web site and a plan What do you suppose is the first step in creating a successful web application using PHP, Apache, and MySQL, after all that planning? We’ll give you a clue: Look at the title of this chapter You need to build the database this site will be based... $e_number “\n”; = wordwrap($msg, 75); switch($error_type) { case E_ERROR: mail(‘admin@example.com’, ‘Fatal Error from Website’, $msg); die(); break; case E_WARNING: mail(‘admin@example.com’, ‘Warning from Website’, $msg); 255 c09.indd 255 12/10/08 5:59:32 PM Part I: Movie Review Web Site break; } } //set error handling to 0 because we will handle all error reporting and //notify admin on warnings... error by using the following code: This produces the following output: Beginning Fatal error: Call to undefined... time for us to push you out of the nest and let you begin to fly on your own So, in this chapter, you will have the opportunity to create your own databases and your own web site We show you how to put together a comic book appreciation web site, but you can take the concepts we teach you and branch off to create that online auction or antique car site you have always dreamed about doing This chapter... normalization ❑ Creating your database ❑ Creating and modifying tables in your database ❑ Building web pages to access your data with PHP Getting Star ted You have a great idea for a site, right? Excellent But don’t open up your PHP editor and start coding just yet! Believe it or not, many people approach the creation of a web site in just this way You may be tempted to do this yourself And while it is not impossible... a plan We’re not going to tell you how to plan out an entire web site, complete with charts and maps and business models That’s not what this book is about We are going to assume that you or somebody in your company has already learned that by reading other great books on business models, attending seminars, reading great articles on the web, and perhaps even hiring a business consultant to help with... finished, using what you have learned, create a little error-catching script to catch the errors 1 2 3 . 2. Enter the following code: < html > < head > < title > Beginning PHP6, Apache, MySQL Web Development Custom Error Page < /title > < /head > < body. it the code highlighted here: < html > < head > < title > Beginning PHP6, Apache, MySQL Web Development Custom Error Page < /title > < /head > < body. to trap errors and customize the error messages, you can use the Apache web server to help do this. How the Apache Web Server Deals with Errors Apache has a directive, the ErrorDocument