Detailed explanation of error handling and exception handling mechanisms in PHP

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Reprinted from: http://www. cnblogs.com/52php/p/5665495.html

When writing PHP programs, error handling is an important part. A program that lacks error detection code looks unprofessional and opens the door to security risks.

Example:

那么正确的写法应该如下：

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<?php $a = fopen(&#39;test.txt&#39;,&#39;r&#39;); //这里并没有对文件进行判断就打开了，如果文件不存在就会报错 ?>

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<?php if (file_exists(&#39;test.txt&#39;)) { $f = fopen(&#39;test.txt&#39;, &#39;r&#39;); // 使用完后关闭 fclose($f); } ?>

1. Three ways of PHP error handling

A. Simple die() statement;

Equivalent to exit( );

example 1

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6

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if (!file_exists('aa.txt')) {
    die('文件不存在');
} else {
    // 执行操作
}
// 如果上面die()被触发，那么这里echo接不被执行
echo 'ok';

简洁写法：

1 2	file_exits('aaa.txt') or die('文件不存在'); echo 'ok';

B. Custom errors and error triggers

1. Error handler (custom errors, generally used for syntax error handling)

Create a custom error function (handler), which must be capable of processing at least two parameters (error_level and errormessage), but can accept up to five parameters (error_file, error_line, error_context)

Syntax ：

function error_function($error_level, $error_message, $error_file, $error_line, $error_context)

// After creation, you need to rewrite set_error_handler(); function

set_error_handler('error_function', E_WARNING); // Here error_function corresponds to the custom handler name created above, and the second parameter is the error level using the custom error handler;

Error reporting level (Just know it)

These error reporting levels are different types of errors that the error handler is designed to handle:

Value	Constant	Description
2	E_WARNING	Non-fatal run-time error. Do not pause script execution.
8	E_NOTICE	Run-time notification. Script discovery errors can occur, but they can also occur while the script is running normally.
256	E_USER_ERROR	Fatal user-generated error. This is similar to E_ERROR set by the programmer using the PHP function trigger_error().
512	E_USER_WARNING	Non-fatal user-generated warning. This is similar to the E_WARNING set by the programmer using the PHP function trigger_error().
1024	E_USER_NOTICE	User-generated notification. This is similar to E_NOTICE set by the programmer using the PHP function trigger_error().
4096	E_RECOVERABLE_ERROR	Catchable fatal error. Like E_ERROR, but can be caught by a user-defined handler. (See set_error_handler())
8191	E_ALL	All errors and warnings except level E_STRICT. (In PHP 6.0, E_STRICT is part of E_ALL)

2、错误触发器（一般用于处理逻辑上的错误）

需求：比如要接收一个年龄，如果数字大于120，就认为是一个错误

传统方法：

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<?php if ($age > 120) { echo &#39;年龄错误&#39;; exit(); } ?>

Use triggers:

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<?php if ($age > 120) { // trigger_error(&#39;错误信息&#39;[，&#39;错误等级&#39;]); 这里错误等级为可选项，用于定义该错误的级别 // 用户定义的级别包含以下三种：E_USER_WARNING 、E_USER_ERROR 、E_USER_NOTICE trigger_error(&#39;年龄错误&#39;); // 这里是调用的系统默认的错误处理方式，我们也可以用自定义处理器 } /** * 自定义处理器，与上面相同 */ function myerror($error_level, $error_message) { echo &#39;error text&#39;; } // 同时需要改变系统默认的处理函数 set_error_handler(&#39;myerror&#39;, E_USER_WARNING); // 同上面，第一个参数为自定义函数的名称，第二个为错误级别【这里的错误级别通常为以下三种：E_USER_WARNING 、E_USER_ERROR 、E_USER_NOTICE】 // 现在再使用trigger_error就可以使用自定义的错误处理函数了 ?>

Exercise questions:

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<?php date_default_timezone_set(&#39;PRC&#39;); function myerror($error_level, $error_message) { $info = "错误号：$error_level"; $info .= "错误信息：$error_message"; $info .= &#39;发生时间：&#39; . date(&#39;Y-m-d H:i:s&#39;); $filename = &#39;aa.txt&#39;; if (!$fp = fopen($filename, &#39;a&#39;)) { echo &#39;创建文件&#39; . $filename . &#39;失败&#39;; } if (is_writeable($filename)) { if (!fwrite($fp, $info)) { echo &#39;写入文件失败&#39;; } else { echo &#39;已成功记录错误信息&#39;; } fclose($fp); } else { echo &#39;文件&#39; . $filename . &#39;不可写&#39;; } exit(); } set_error_handler(&#39;myerror&#39;, E_WARNING); $fp = fopen(&#39;aaa.txt&#39;, &#39;r&#39;); ?>

C. Error log

By default, according to the error_log configuration in php.ini, PHP sends error records to the server's error recording system or file. Error records can be sent to a file or remote destination by using the error_log() function;

Syntax:

error_log(error[, type, destination, headers])

type The part generally uses 3, which means appending error information after the file without overwriting the original content. destination means the destination, that is, the stored file or remote destination

For example: error_log("$error_info",3," errors.txt");

2. PHP exception handling [Key points]

1. Basic syntax

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<?php try { // 可能出现错误或异常的代码 //catch 捕获 Exception是PHP已定义好的异常类 } catch (Exception $e) { // 对异常处理，方法： //1、自己处理 //2、不处理，可以再次抛出 throw new Exception(&#39;xxx&#39;); } ?>

2. The handler should include:

1. try - The function that uses exceptions should be located within the "try" code block. If no exception is triggered, the code continues execution as usual. But if an exception is triggered, an exception will be thrown;

2. throw - This specifies how to trigger the exception. Each "throw" must correspond to at least one "catch";

3. catch - The "catch" code block will catch the exception and create an object containing the exception information;

Let’s trigger an exception:

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<?php /** * 创建可抛出一个异常的函数 */ function checkNum($number) { if ($number > 1) { throw new Exception("Value must be 1 or below"); } return true; } // 在 "try" 代码块中触发异常 try { checkNum(2); // 如果异常被抛出，那么下面一行代码将不会被输出 echo &#39;If you see this, the number is 1 or below&#39;; } catch (Exception $e) { // 捕获异常 echo &#39;Message: &#39; . $e->getMessage(); } ?>

上面代码将获得类似这样一个错误：

1	Message: Value must be 1 or below

Explanation of example:

The above code throws an exception and catches it:

1. Create a checkNum() function that checks whether a number is greater than 1 and, if so, throws an exception.

2. Call the checkNum() function in the "try" code block.

3. Exception in checkNum() function is thrown.

4. The "catch" code block receives the exception and creates an object ($e) containing the exception information.

5. Output the error message from this exception by calling $e->getMessage() from this exception object.

However, in order to follow the principle of "each throw must correspond to a catch", you can set up a top-level exception handler to handle missed errors.

The set_exception_handler() function can set a user-defined function that handles all uncaught exceptions.

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<?php /** * 设置一个顶级异常处理器 */ function myexception($e) { echo &#39;this is top exception&#39;; } // 修改默认的异常处理器 set_exception_handler("myexception"); try { $i = 5; if ($i < 10) { throw new Exception(&#39;$i must greater than 10&#39;); } } catch (Exception $e) { // 处理异常 echo $e->getMessage() . &#39;<br/>&#39;; // 不处理异常，继续抛出 throw new Exception(&#39;errorinfo&#39;); // 也可以用throw $e 保留原错误信息; } ?>

Create a custom exception class

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<?php class customException extends Exception { public function errorMessage() { $errorMsg = &#39;Error on line &#39; . $this->getLine() . &#39; in &#39; . $this->getFile() . &#39;: <b>&#39; . $this->getMessage() . &#39;</b> is not a valid E-Mail address&#39;; return $errorMsg; } } // 使用 try { throw new customException(&#39;error message&#39;); } catch (customException $e) { echo $e->errorMessage(); } ?>

You can use multiple catches to return error messages under different circumstances

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<?php try { $i = 5; if ($i > 0) { throw new customException(&#39;error message&#39;); // 使用自定义异常类处理 } if ($i < -10) { throw new Exception(&#39;error2&#39;); // 使用系统默认异常处理 } } catch (customException $e) { echo $e->getMessage(); } catch (Exception $e1) { echo $e1->getMessage(); } ?>

异常的规则

• 需要进行异常处理的代码应该放入 try 代码块内，以便捕获潜在的异常。

• 每个try或throw代码块必须至少拥有一个对应的 catch 代码块。

• 使用多个 catch 代码块可以捕获不同种类的异常。

• 可以在try代码内的catch 代码块中再次抛出（re-thrown）异常。

简而言之：如果抛出了异常，就必须捕获它。

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