Can I try/catch a warning?

I need to catch some warnings being thrown from some php native functions and then handle them.

Specifically:

array dns_get_record  ( string $hostname  [, int $type= DNS_ANY  [, array &$authns  [, array &$addtl  ]]] )

It throws a warning when the DNS query fails.

try/catch doesn't work because a warning is not an exception.

I now have 2 options:

1. set_error_handler seems like overkill because I have to use it to filter every warning in the page (is this true?);

2. Adjust error reporting/display so these warnings don't get echoed to screen, then check the return value; if it's false, no records is found for hostname.

What's the best practice here?

Set and restore error handler

One possibility is to set your own error handler before the call and restore the previous error handler later with restore_error_handler().

set_error_handler(function() { /* ignore errors */ });
dns_get_record();
restore_error_handler();

You could build on this idea and write a re-usable error handler that logs the errors for you.

set_error_handler([$logger, 'onSilencedError']);
dns_get_record();
restore_error_handler();

Turning errors into exceptions

You can use set_error_handler() and the ErrorException class to turn all php errors into exceptions.

set_error_handler(function($errno, $errstr, $errfile, $errline) {
    // error was suppressed with the @-operator
	if (0 === error_reporting()) {
	    return false;
	}
	
	throw new ErrorException($errstr, 0, $errno, $errfile, $errline);
});

try {
    dns_get_record();
} catch (ErrorException $e) {
    // ...
}

The important thing to note when using your own error handler is that it will bypass the error_reporting setting and pass all errors (notices, warnings, etc.) to your error handler. You can set a second argument on set_error_handler() to define which error types you want to receive, or access the current setting using ... = error_reporting() inside the error handler.

Suppressing the warning

Another possibility is to suppress the call with the @ operator and check the return value of dns_get_record() afterwards. But I'd advise against this as errors/warnings are triggered to be handled, not to be suppressed.

The solution that really works turned out to be setting simple error handler with E_WARNING parameter, like so:

set_error_handler("warning_handler", E_WARNING);
dns_get_record(...)
restore_error_handler();

function warning_handler($errno, $errstr) { 
// do something
}

Be careful with the @ operator - while it suppresses warnings it also suppresses fatal errors. I spent a lot of time debugging a problem in a system where someone had written @mysql_query( '...' ) and the problem was that mysql support was not loaded into PHP and it threw a silent fatal error. It will be safe for those things that are part of the PHP core but please use it with care.

bob@mypc:~$ php -a
Interactive shell

php > echo @something(); // this will just silently die...

No further output - good luck debugging this!

bob@mypc:~$ php -a
Interactive shell

php > echo something(); // lets try it again but don't suppress the error
PHP Fatal error:  Call to undefined function something() in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
bob@mypc:~$ 

This time we can see why it failed.

I wanted to try/catch a warning, but at the same time keep the usual warning/error logging (e.g. in /var/log/apache2/error.log); for which the handler has to return false. However, since the "throw new..." statement basically interrupts the execution, one then has to do the "wrap in function" trick, also discussed in:

https://stackoverflow.com/questions/4905867/is-there-a-static-way-to-throw-exception-in-php/4905889#4905889

Or, in brief:

  function throwErrorException($errstr = null,$code = null, $errno = null, $errfile = null, $errline = null) {
    throw new ErrorException($errstr, 0, $errno, $errfile, $errline);
  }
  function warning_handler($errno, $errstr, $errfile, $errline, array $errcontext) {
    return false && throwErrorException($errstr, 0, $errno, $errfile, $errline);
    # error_log("AAA"); # will never run after throw
    /* Do execute PHP internal error handler */
    # return false; # will never run after throw
  }
  ...
  set_error_handler('warning_handler', E_WARNING);
  ...
  try {
    mkdir($path, 0777, true);
  } catch (Exception $e) {
    echo $e->getMessage();
    // ...
  }

---

EDIT: after closer inspection, it turns out it doesn't work: the "return false && throwErrorException ... " will, basically, not throw the exception, and just log in the error log; removing the "false &&" part, as in "return throwErrorException ...", will make the exception throwing work, but will then not log in the error_log... I'd still keep this posted, though, as I haven't seen this behavior documented elsewhere.

Normaly you should never use @ unless this is the only solution. In that specific case the function dns_check_record should be use first to know if the record exists.

Combining these lines of code around a file_get_contents() call to an external url helped me handle warnings like "failed to open stream: Connection timed out" much better:

set_error_handler(function ($err_severity, $err_msg, $err_file, $err_line, array $err_context)
{
    throw new ErrorException( $err_msg, 0, $err_severity, $err_file, $err_line );
}, E_WARNING);
try {
    $iResult = file_get_contents($sUrl);
} catch (Exception $e) {
    $this->sErrorMsg = $e->getMessage();
}
restore_error_handler();

This solution works within object context, too. You could use it in a function:

public function myContentGetter($sUrl)
{
  ... code above ...
  return $iResult;
}

You should probably try to get rid of the warning completely, but if that's not possible, you can prepend the call with @ (i.e. @dns_get_record(...)) and then use any information you can get to figure out if the warning happened or not.

If dns_get_record() fails, it should return FALSE, so you can suppress the warning with @ and then check the return value.

try checking whether it returns some boolean value then you can simply put it as a condition. I encountered this with the oci_execute(...) which was returning some violation with my unique keys.

ex.
oci_parse($res, "[oracle pl/sql]");
if(oci_execute){
...do something
}

FolderStructure

index.php //Script File
logs //Folder for log Every warning and Errors
CustomException.php //Custom exception File

CustomException.php

/**
* Custom error handler
*/
function handleError($code, $description, $file = null, $line = null, $context = null) {
    $displayErrors = ini_get("display_errors");;
    $displayErrors = strtolower($displayErrors);
    if (error_reporting() === 0 || $displayErrors === "on") {
        return false;
    }
    list($error, $log) = mapErrorCode($code);
    $data = array(
        'timestamp' => date("Y-m-d H:i:s:u", time()),
        'level' => $log,
        'code' => $code,
        'type' => $error,
        'description' => $description,
        'file' => $file,
        'line' => $line,
        'context' => $context,
        'path' => $file,
        'message' => $error . ' (' . $code . '): ' . $description . ' in [' . $file . ', line ' . $line . ']'
    );
    $data = array_map('htmlentities',$data);
    return fileLog(json_encode($data));
}

/**
* This method is used to write data in file
* @param mixed $logData
* @param string $fileName
* @return boolean
*/
function fileLog($logData, $fileName = ERROR_LOG_FILE) {
    $fh = fopen($fileName, 'a+');
    if (is_array($logData)) {
        $logData = print_r($logData, 1);
    }
    $status = fwrite($fh, $logData . "\n");
    fclose($fh);
//    $file = file_get_contents($filename);
//    $content = '[' . $file .']';
//    file_put_contents($content); 
    return ($status) ? true : false;
}

/**
* Map an error code into an Error word, and log location.
*
* @param int $code Error code to map
* @return array Array of error word, and log location.
*/
function mapErrorCode($code) {
    $error = $log = null;
    switch ($code) {
        case E_PARSE:
        case E_ERROR:
        case E_CORE_ERROR:
        case E_COMPILE_ERROR:
        case E_USER_ERROR:
            $error = 'Fatal Error';
            $log = LOG_ERR;
            break;
        case E_WARNING:
        case E_USER_WARNING:
        case E_COMPILE_WARNING:
        case E_RECOVERABLE_ERROR:
            $error = 'Warning';
            $log = LOG_WARNING;
            break;
        case E_NOTICE:
        case E_USER_NOTICE:
            $error = 'Notice';
            $log = LOG_NOTICE;
            break;
        case E_STRICT:
            $error = 'Strict';
            $log = LOG_NOTICE;
            break;
        case E_DEPRECATED:
        case E_USER_DEPRECATED:
            $error = 'Deprecated';
            $log = LOG_NOTICE;
            break;
        default :
            break;
    }
    return array($error, $log);
}
//calling custom error handler
set_error_handler("handleError");

just include above file into your script file like this

index.php

error_reporting(E_ALL);
ini_set('display_errors', 'off');
define('ERROR_LOG_FILE', 'logs/app_errors.log');

include_once 'CustomException.php';
echo $a; // here undefined variable warning will be logged into logs/app_errors.log

As of PHP8, you can do the following instead of setting error handlers to catch Errors and Warnings. I Believe in PHP 7.something you could catch some Errors.

try {
    call_user_func('sprintf', array_merge([$string], $args));
} catch (Throwable $e) {
    $logger->info('mesage...');
}

You should generally be somewhere that you can pass or access a logger if you bulkhead in this way, as it can obfuscate coder errors, such as passing incorrectly typed parameters to a method, and mask a variety of other problems.

https://php.watch/versions/8.0/internal-function-exceptions

Not sure if notices are caught (likely not), but you can likely solve around examples like this one, by thinking a little more about what you are looking to do.

Both builder pattern, and options patterns provide solutions for this where prior to the site of call, which can be a private function or just after validity checks, you can throw a real custom exception that is attributable only to your code. That will make even built-in functions very safe to use.

One other nice practice is to use either debug_backtrace, with DEBUG_BACKTRACE_IGNORE_ARGS or use the getTrace or getTraceAsString methods on the Throwable so that some of the context is preseved.

I would only recommend using @ to suppress warnings when it's a straight forward operation (e.g. $prop = @($high/($width - $depth)); to skip division by zero warnings). However in most cases it's better to handle.