About PHP exception handling operations

Exception is used to change the normal flow of the script when a specified error occurs. This article will explain in detail the operation of php exception handling.

What is an exception?

PHP 5 provides a new object-orientederror handling method.

Exception handling is used to change the normal flow of the script when a specified error (exception) situation occurs. This situation is called an exception.

When an exception is triggered, what usually happens is:

The current code state is saved

Code execution is switched to the predefined exception handler function

Depending on the situation, the processor may restart code execution from the saved code state, terminate script execution, or continue script execution from another location in the code

We will show different error handling methods:

Basic use of exceptions

Create custom exception handlers

Multiple exceptions

Re-throw exceptions

Set top-level exception handler

Basic use of exceptions

When an exception is thrown, the subsequent code will not continue to execute, and PHP will try to find a matching "catch" code block.

If the exception is not caught and there is no need to use

set_exception_handler() for corresponding processing, then a serious error (fatal error) will occur and "Uncaught Exception" will be output (Uncaught Exception) error message.

Let's try throwing an exception without catching it:

<?php
//create function with an exception
function checkNum($number)
 {
 if($number>1)
  {
  throw new Exception("Value must be 1 or below");
  }
 return true;
 }
//trigger exception
checkNum(2);
?>

The above code will get an error like this:

Fatal error: Uncaught exception ' Exception'

with message 'Value must be 1 or below' in C:\webfolder\test.php:6
Stack trace: #0 C:\webfolder\test.php(12):
checkNum (28) #1 {main} thrown in C:\webfolder\test.php on line 6

Try, throw and catch

To avoid the errors in the above example, we Appropriate code needs to be created to handle exceptions.

The correct handler should include:

Try - The function that uses the exception 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.

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

Catch - The "catch" code block will catch the exception and create an object containing the exception information

Let's trigger an exception:

//Create a function that can throw an exception

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); //If the exception is thrown, this text will not be shown echo 'If you see this, the number is 1 or below';
   }//捕获异常catch(Exception $e)
    { echo 'Message: ' .$e->getMessage();
     }?>

The above code will get an error similar to this:

Message: Value must be 1 or below

Example explanation:

The above code throws an exception and catches it:

Create checkNum() function. It detects whether the number is greater than 1. If so, throw an exception.

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

Exception in checkNum() function is thrown

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

By calling $e->getMessage() from this exception object, the error message from the exception is output.

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

Create a custom Exception class

Creating a custom exception handler is very simple. We simply created a specialized class whose functions are called when an exception occurs in PHP. This class must be an extension of the exception class.

This custom exception class inherits all properties of PHP's exception class, and you can add custom functions to it.

We start by creating the exception class:

<?php
class customException extends Exception
 {
 public function errorMessage()
  {
  //error message
  $errorMsg = &#39;Error on line &#39;.$this->getLine().&#39; in &#39;.$this->getFile()
  .&#39;: <b>&#39;.$this->getMessage().&#39;</b> is not a valid E-Mail address&#39;;
  return $errorMsg;
  }
 }
$email = "someone@example...com";
try
 {
 //check if 
 if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE)
  {
  //throw exception if email is not valid
  throw new customException($email);
  }
 }
catch (customException $e)
 {
 //display custom message
 echo $e->errorMessage();
 }
?>

This new class is a copy of the old exception class, plus the errorMessage() function. Just because it is a copy of the old class, it inherits

properties and methods from the old class, and we can use the exception class methods, such as getLine(), getFile() and getMessage().

Explanation of example:

The above code throws an exception and catches it through a custom exception class:

The customException() class is as old Created as an extension of the exception class. This way it inherits all properties and methods of the old class.

Create errorMessage() function. If the e-mail address is illegal, the function returns an error message

Set the $email variable to the illegal e-mail address string

Execute the "try" code block, because The e-mail address is invalid, so an exception is thrown

The "catch" code block catches the exception and displays the error message

Multiple exceptions

Can be used for a script Multiple exceptions to detect multiple situations.

可以使用多个 if..else 代码块，或一个 switch 代码块，或者嵌套多个异常。这些异常能够使用不同的 exception 类，并返回不同的错误消息：

<?php
class customException extends Exception
{
public function errorMessage()
{
//error message
$errorMsg = &#39;Error on line &#39;.$this->getLine().&#39; in &#39;.$this->getFile()
.&#39;: <b>&#39;.$this->getMessage().&#39;</b> is not a valid E-Mail address&#39;;
return $errorMsg;
}
}
$email = "someone@example.com";
try
 {
 //check if 
 if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE)
  {
  //throw exception if email is not valid
  throw new customException($email);
  }
 //check for "example" in mail address
 if(strpos($email, "example") !== FALSE)
  {
  throw new Exception("$email is an example e-mail");
  }
 }
catch (customException $e)
 {
 echo $e->errorMessage();
 }
catch(Exception $e)
 {
 echo $e->getMessage();
 }
?>

例子解释：

上面的代码测试了两种条件，如何任何条件不成立，则抛出一个异常：

customException() 类是作为旧的 exception 类的一个扩展来创建的。这样它就继承了旧类的所有属性和方法。

创建 errorMessage() 函数。如果 e-mail 地址不合法，则该函数返回一个错误消息。

执行 "try" 代码块，在第一个条件下，不会抛出异常。

由于 e-mail 含有字符串 "example"，第二个条件会触发异常。

"catch" 代码块会捕获异常，并显示恰当的错误消息

如果没有捕获 customException，紧紧捕获了 base exception，则在那里处理异常。

重新抛出异常

有时，当异常被抛出时，您也许希望以不同于标准的方式对它进行处理。可以在一个 "catch" 代码块中再次抛出异常。

脚本应该对用户隐藏系统错误。对程序员来说，系统错误也许很重要，但是用户对它们并不感兴趣。为了让用户更容易使用，您可以再次抛出带有对用户比较友好的消息的异常：

<?php
class customException extends Exception
 {
 public function errorMessage()
  {
  //error message
  $errorMsg = $this->getMessage().&#39; is not a valid E-Mail address.&#39;;
  return $errorMsg;
  }
 }
$email = "someone@example.com";
try
 {
 try
  {
  //check for "example" in mail address
  if(strpos($email, "example") !== FALSE)
   {
   //throw exception if email is not valid
   throw new Exception($email);
   }
  }
 catch(Exception $e)
  {
  //re-throw exception
  throw new customException($email);
  }
 }
catch (customException $e)
 {
 //display custom message
 echo $e->errorMessage();
 }
?>

例子解释：

上面的代码检测在邮件地址中是否含有字符串 "example"。如果有，则再次抛出异常：

customException() 类是作为旧的 exception 类的一个扩展来创建的。这样它就继承了旧类的所有属性和方法。

创建 errorMessage() 函数。如果 e-mail 地址不合法，则该函数返回一个错误消息。

把 $email 变量设置为一个有效的邮件地址，但含有字符串 "example"。

"try" 代码块包含另一个 "try" 代码块，这样就可以再次抛出异常。

由于 e-mail 包含字符串 "example"，因此触发异常。

"catch" 捕获到该异常，并重新抛出 "customException"。

捕获到 "customException"，并显示一条错误消息。

如果在其目前的 "try" 代码块中异常没有被捕获，则它将在更高层级上查找 catch 代码块。

设置顶层异常处理器 （Top Level Exception Handler）

set_exception_handler() 函数可设置处理所有未捕获异常的用户定义函数。

<?php
function myException($exception)
{
echo "<b>Exception:</b> " , $exception->getMessage();
}
set_exception_handler(&#39;myException&#39;);
throw new Exception(&#39;Uncaught Exception occurred&#39;);
?>

以上代码的输出应该类似这样：

Exception: Uncaught Exception occurred

在上面的代码中，不存在 "catch" 代码块，而是触发顶层的异常处理程序。应该使用此函数来捕获所有未被捕获的异常。

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 代码块内的 catch 代码块中再次抛出（re-thrown）异常。

本篇详细的讲解了对于php异常的处理，更多的学习资料清关注php中文网即可观看。

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