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Error handling and debugging
- 1. General Error Types and Debugging Pengaturcaraan PHP Pengaturcaraan PHP When developing Web applications with PHP and MySQL, you end up with potential bugs in one of four or more technologies. You could have HTML issues, PHP problems, SQL errors, or MySQL mistakes. To be able to fix the bug, you must first determine in what realm the bug resides. 1
- 2. Pengaturcaraan PHP HTML problems are often the least disruptive and the easiest to catch. You normally know there's a problem when your layout is all messed up. PHP errors are the ones you'll see most often, as this language will be at the heart of your applications. PHP errors fall into three general areas: Logical Run time Syntactical Pengaturcaraan PHP 2
- 3. Pengaturcaraan PHP Pengaturcaraan PHP SQL errors are normally a matter of syntax, and they'll be reported when you try to run the query on MySQL. For example, this is a common error: The syntax is just wrong, confused with the SELECT syntax (SELECT * FROM tablename). The right syntax is: 3
- 4. Pengaturcaraan PHP Pengaturcaraan PHP Let's examine how to begin debugging any problem. Step 1 Make sure that you are running the right page. It's altogether too common that you try to fix a problem and no matter what you do, it never goes away. The reason: you've actually been editing a different page than you thought. 4
- 5. Pengaturcaraan PHP Step 2 Make sure that you have saved your latest changes. Step 3 Make sure that you run all PHP pages through the URL. Because PHP requires the use of a Web server, running any PHP code requires that you access the page through a URL (http://www.sitename.com/page.php or http://localhost/page.php). If you double-click a PHP page to open it in a browser (or use the browser's File | Open option), you'll see the PHP code, not the executed result. Pengaturcaraan PHP 5
- 6. Pengaturcaraan PHP Pengaturcaraan PHP Step 5 Know what Web server you are running. Similarly, some problems and features are unique to your Web serving application — Apache, IIS, and PWS. You should know which one you are using, and which version, from when you installed the application. 6
- 7. Pengaturcaraan PHP Step 6 Try executing pages in a different Web browser. Every Web developer should have and use at least two Web browsers. If you test your pages in different ones, you'll see if the problem has to do with your script or a particular browser. Debugging HTML Pengaturcaraan PHP 7
- 9. Pengaturcaraan PHP Step 5 Test the page in another browser. PHP code is generally browser- independent, meaning you'll get consistent results regardless of the client. Not so with HTML. Sometimes a particular browser has a quirk that affects the rendered page. Running the same page in another browser is the easiest way to know if it's an HTML problem or a browser quirk. Displaying PHP Errors Pengaturcaraan PHP 9
- 10. Pengaturcaraan PHP Pengaturcaraan PHP To turn on display_errors in a script, use the ini_set() function. As its arguments, this function takes a directive name and what setting it should have: Including this line in a script will turn on display_errors for that script. The only downside is that if your script has a syntax error that prevents it from running at all, then you'll still see a blank page. To have PHP display errors for the entire server, edit your php.ini file, assuming you can (unless you host your site on your own computer, it probably won't be an option). 10
- 12. Creating Custom Error Handlers Pengaturcaraan PHP Pengaturcaraan PHP Adjusting error reporting is one option for error management with your sites, and another is to alter how PHP handles errors. You can create your own functions that will overrule PHP's default behavior (printing them out in HTML tags). For example, The set_error_handler() function determines what function handles errors. Feeding this function the name of another function will result in that second function (e.g., report_errors) being called whenever an error is encountered. This function (report_errors) will, at that time, receive two arguments — an error number and a textual error message — which can be used in any possible manner. 12
- 13. Pengaturcaraan PHP This function can also be written to take up to five arguments, the extra three being the name of the file where the error was found, the specific line number, and the variables that existed at the time of the error. Logging PHP Errors Pengaturcaraan PHP 13
- 14. Pengaturcaraan PHP In addition to handling errors by printing them out, PHP can also log the errors to make a permanent note of them. For this purpose, the error_log() function instructs PHP how to file an error. The message value should be the text of the logged error. The type dictates how the error is logged. The destination parameter can be either the name of a file (for log type 3) or an email address (for log type 1). The extra headers argument is used only when sending emails (log type 1). Both the destination and extra headers are optional. Pengaturcaraan PHP The types of error logging you can use are listed here. Number Meaning 0 The message is logged using the operating system's default method. 1 The message is sent by email to the destination address. 2 The message is sent to a remote debugger (for example, another server). 3 The message is recorded in a text file. 14
- 15. PHP Debugging Techniques Pengaturcaraan PHP 15
- 16. Pengaturcaraan PHP Error Likely Cause Blank Page HTML problem, or PHP error and display_errors is off. Parse error Missing semicolon; unbalanced curly braces, parentheses, or quotation marks; or use of an unescaped quotation mark in a string. Empty variable Forgot the initial $, misspelled or miscapitalized the variable name, value inappropriate variable scope (with functions), or the register_globals setting is turned off. Undefined variable Reference made to a variable before it is given a value or an empty variable value (see those potential causes). Call to undefined Misspelled function name, PHP is not configured to use that function function (like a MySQL function), or the document that contains the function definition is not properly included. Cannot redeclare Two definitions of your own function exist, check within included function files. Headers already White space exists in the script before the PHP tags, data has sent already been printed, or a file has been included. Pengaturcaraan PHP 16
- 17. Pengaturcaraan PHP If PHP reports an error on the last line of your document, this is almost always because a mismatched parenthesis, curly brace, or quotation mark was not caught until that moment. Pengaturcaraan PHP 17
- 18. Pengaturcaraan PHP Pengaturcaraan PHP Use Comments Just as you can use comments to document your scripts, you can also use them to rule out problematic lines. If PHP is giving you an error on line 12, then commenting out that line should get rid of the error. If not, then you know the error is elsewhere. 18
- 19. End Pengaturcaraan PHP 19