Detailed explanation of instance usage of PHP error control operator @ or die()

PHP supports an error control operator: @. When placed before a PHP expression, any error message that expression may produce is ignored.

If you set a custom error handling function with set_error_handler(), it will still be called, but this error handling function can (and should) call error_reporting(), and this function has @ will return 0.

If the track_errors feature is activated, any error messages generated by the expression are stored in the variable $php_errormsg. This variable is overwritten on every error, so check it as early as possible if you want to use it.

php ignore error symbol @

@ is an ignore error character in the PHP language. When placed before a PHP expression, the expression may Any error messages generated are ignored. For example:

@include("file.php");

It should be noted that the @ operator is only valid for expressions. You can think of it this way, if you can get a value from somewhere, you can add the @ operator in front of it.

For example, you can put it before variables, function and include calls, constants, etc.

It cannot be placed before the definition of a function or class, nor can it be used in conditional structures such as if and foreach.

In addition, @ is usually used together with or die(), for example:

(@include("file.php"))
 OR die("Could not find file.php!");

The above statement means that if the inclusion of file.php is unsuccessful, for example, the file.php file does not exist , print "Could not find file.php!" on the web page, and immediately stop the execution of the program.

If the @ character is not added, then when include("file.php") returns an error, what is printed on the web page is:

Warning: include(file.php) [function.include]: failed to open stream: No such file or directory
Warning: include() [function.include]: Failed opening 'file.php' for inclusion (include_path='.;C:\php5\pear')

Some usage of or die()

Everyone is familiar with using the die() function, but you may not be aware of some of the effects produced by writing it like or die().

First understand die().

die definition and usage

die() function outputs a message and exits the current script.

Syntax

die(status)

status Required. Specifies the message or status number to be written before exiting the script. Status numbers are not written to the output.

If status is a string, the function will output the string before exiting.

If status is an integer, this value will be used as the exit status. The exit status value is between 0 and 254. Exit status 255 is reserved by PHP and will not be used. Status 0 is used to terminate the program successfully.

Note: If the version number of PHP is greater than or equal to 4.2.0, then if status is an integer, this parameter will not be output.

php or die() statement

We often see statements like this:

$file = fopen($filename, 'r') or die("抱歉，无法打开: $filename");

or this is how it is understood here, because in PHP Data types are not distinguished, so $file can be either int or bool, so such a statement will not report an error. But some friends may not understand the processing process.

In fact, in most languages, in statements like bool or bool, if the previous value is true, the next value will not be judged. The same is true here, so if the fopen function is executed correctly, it will return an int value greater than 0 (which is actually "true"), and the subsequent statements will not be executed.

If the fopen function fails to execute, it will return false, and then it will be judged whether the following expression is true.

After executing die(), no matter what is returned, the program has stopped executing and the specified error message is displayed, thus achieving the purpose of debugging. that's all.

Two functions commonly used with error management are die() and exit() (strictly speaking, they are language constructs rather than functions, but who cares about that).

When die() and exit() are called in a script, the entire script will be terminated.

They can all be used to prevent the script from continuing to execute, so that certain important operations (such as establishing a database connection) will not occur. You can also pass die() and exit() a string that will be printed in the browser.

You can usually see die() and exit() used in OR conditional statements.

For example

include('config.inc.php') OR die ('Could not open the file. ');

After including such a line of code, if PHP cannot include the configuration file, the die() statement will be executed and the could not open the file message will be printed.

!defined('CURSCRIPT') && define('CURSCRIPT', ");//意思是如果没定义CURSCRIPT常量,则定义CURSCRIPT为空.

You'll see variations of this throughout the book and in the PHP manual, as it's a quick way to handle errors (but potentially overhandling them) without having to use custom errors. handler.

In fact, die and exit are equivalent, used to terminate the current script, often used when debugging programs.

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