Fire-Proofing Your Code
Even the best developers make mistakes sometimes. That's why most programming languages
- including PHP - come with built-in capabilities to catch errors and take remedial
action. This action can be as simple as displaying an error message, or as complex as
sending the site administrator an email with a complete stack trace.
To make it easier to do this, PHP comes with a full-featured error handling API that can
be used to trap and resolve errors. In addition to deciding which types of errors a user
sees, you can also replace the built-in error handling mechanism with your own custom (and
usually more creative) functions. If you're using PHP 5, you get a bonus: a spanking-new
exception model, which lets you wrap your code in Java-like try-catch()
blocks
for more efficient error handling.
In this edition of PHP 101, I'm going to discuss all these things, giving you a crash course
in how to add error-handling to your PHP application. Keep reading - this is pretty cool stuff!
Rogues Gallery
Before we get into the nitty-gritty of how to write an error handler, you need to know a
little theory.
Normally, when a PHP script encounters an error, it displays a message indicating the cause
of the error and may also (depending on how serious the error is) terminate script execution.
Now, while this behaviour is acceptable during the development phase, it cannot continue once
a PHP application has been released to actual users. In "live" situations, it is unprofessional
to display cryptic error messages (which are usually incomprehensible to non-technical users);
it is more professional to intercept these errors and either resolve them (if resolution is
possible), or notify the user with an easily-understood error message (if not).
There are three basic types of runtime errors in PHP:
- Notices: These are trivial, non-critical errors that PHP encounters while
executing a script - for example, accessing a variable that has not yet been defined.
By default, such errors are not displayed to the user at all - although, as you will see,
you can change this default behaviour.
- Warnings: These are more serious errors - for example, attempting to
include()
a file which does not exist. By default, these errors are displayed
to the user, but they do not result in script termination. - Fatal errors: These are critical errors - for example, instantiating an object
of a non-existent class, or calling a non-existent function. These errors cause the
immediate termination of the script, and PHP's default behaviour is to display them to the
user when they take place.
It should be noted that a syntax error in a PHP script - for example, a missing brace
or semi-colon - is treated as a fatal error and results in script termination. That's
why, if you forget a semi-colon at the end of one of your PHP statements, PHP will refuse
to execute your script until you correct the mistake.
PHP errors can be generated by the Zend engine, by PHP built-in functions, or by user-defined
functions. They may occur at startup, at parse-time, at compile-time or at run-time.
Internally, these variations are represented by twelve different error types (as of PHP 5),
and you can read about them at
http://www.php.net/manual/en/ref.errorfunc.php. Named constants like
E_NOTICE
and E_USER_ERROR
provide a convenient way to reference
the different error types.
A quick tip here: most of the time, you'll be worrying about run-time errors
(E_NOTICE
, E_WARNING
and E_ERROR
) and
user-triggered errors (E_USER_NOTICE
, E_USER_WARNING
and
E_USER_ERROR
). During the debug phase, you can use the shortcut
E_ALL
type to see all fatal and non-fatal errors generated by your script,
and in PHP 5 you might also want to use the new E_STRICT
error type to view
errors that affect the forward compatibility of your code.
Early Warning
With the theory out of the way, let's now apply it to some examples. Consider the
following code snippet:
<?php
// initialize the $string variable
$string = 'a string';
// explode() a string
// this will generate a warning or E_WARNING because the number of arguments to explode() is incorrect
explode($string);
?>
If you run this script, you'll get a non-fatal error (E_WARNING
), which
means that if you had statements following the call to explode()
, they
would still get executed. Try it for yourself and see!
To generate a fatal error, you need to put in a bit more work. Take a look at this:
<?php
// call a non-existent function
// this will generate a fatal error (E_ERROR)
callMeJoe();
?>
Here, the call to a non-existent function trips all of PHP's alarm wires and generates a
fatal error, which immediately stops script execution.
Now, here's the interesting bit. You can control which errors are displayed to the user,
by using a built-in PHP function called error_reporting()
. This
function accepts a named constant, and tells the script to report only errors that
match that type. To see this in action, consider the following rewrite of one of the
earlier scripts to "hide" non-fatal errors:
<?php
// report only fatal errors
error_reporting(E_ERROR);
// initialize the $string variable
$string = 'string';
// attempt to explode() a string
// this will not generate a warning because only fatal errors are reported
explode($string);
?>
In this case, when the script executes, no warning will be generated even though the call
to explode()
contains one less argument than it should.
You can use a similar technique to turn off the display of fatal errors:
<?php
// report no fatal errors
error_reporting(~E_ERROR);
// call a non-existent function
callMeJoe();
?>
Keep in mind, though, that just because the error isn't being reported doesn't mean it
isn't occurring. Although the script above will not display a visible error message, script
execution will still stop at the point of error and statements subsequent to that point will
not be executed. error_reporting()
gives you control over which errors are
displayed; it doesn't prevent the errors themselves.
Note that there are further settings within php.ini that should be used on production
sites. You can (and should) turn off display_errors
, stipulate an
error_log
file and switch on log_errors
.
Note also that the approach used above to hide error messages, although extremely simple, is
not recommended for general use. It is poor programming practice to trap all errors,
regardless of type, and ignore them; it is far better - and more professional - to anticipate
the likely errors ahead of time, and write defensive code that watches for them and handles
them appropriately. This will prevent your users from finding themselves staring at an
unexplained blank page when something goes wrong.
Rolling Your Own
With this in mind, let's talk a little bit about changing the way errors are handled.
Consider a typical PHP error message: it lists the error type, a descriptive message,
and the name of the script that generated the error. Most of the time, this is more
than sufficient... but what if your boss is a demanding customer, and insists that
there must be a "better way"?
Well, there is. It's a little function called set_error_handler()
, and it
allows you to divert all PHP errors to a custom function that you've defined, instead
of sending them to the default handler. This custom function must be capable of
accepting a minimum of two mandatory arguments (the error type and corresponding
descriptive message) and up to three additional arguments (the file name and line
number where the error occurred and a dump of the variable space at the time of error).
The following example might make this clearer:
<?php
// define a custom error handler
set_error_handler('oops');
// initialize the $string variable
$string = 'a string';
// explode() a string
// this will generate a warning because the number of arguments to explode() is incorrect
// the error will be caught by the custom error handler
explode($string);
// custom error handler
function oops($type, $msg, $file, $line, $context) {
echo "<h1>Error!</h1>";
echo "An error
occurred while executing this script. Please contact the <a
href=mailto:webmaster@somedomain.com>webmaster</a> to report
this error.";
echo "<p />";
echo "Here is the information provided by the script:";
echo "<hr><pre>";
echo "Error code: $type<br />";
echo "Error message: $msg<br />";
echo "Script name and line number of error: $file:$line<br />";
$variable_state = array_pop($context);
echo "Variable state when error occurred: ";
print_r($variable_state);
echo "</pre><hr>";
}
?>
The set_error_handler()
function tells the script that all errors are to
be routed to my user-defined oops()
function. This function is set up to
accept five arguments: error type, message, file name, line number, and an array
containing a lot of information about the context that the error occurred in (including
server and platform, as well as script information). The final element of the context
array contains the current value of the guilty variable. These arguments are then used
to create an error page that is friendlier and more informative than PHP's standard
one-line error message.
You can use this custom error handler to alter the error message the user sees, on the
basis of the error type. Take a look at the next example, which demonstrates this technique:
<?php
// define a custom error handler
set_error_handler('oops');
// initialize $string variable
$string = 'a string';
// this will generate a warning
explode($string);
// custom error handler
function oops($type, $msg, $file, $line, $context) {
switch ($type) {
// notices
case E_NOTICE:
// do nothing
break;
// warnings
case E_WARNING:
// report error
print "Non-fatal error on line $line of $file: $msg <br />";
break;
// other
default:
print "Error of type $type on line $line of $file: $msg <br />";
break;
}
}
?>
Note that certain error types can't be handled in this way. For example, a fatal
E_ERROR
will prevent the PHP script from continuing, therefore it can
never reach a user-created error-handling mechanism. See http://www.php.net/set-error-handler for more information on this.
Pulling the Trigger
So far we've been talking about handling errors generated by PHP itself, but why stop
there? PHP allows you to use its built-in error handling system to raise your own custom
errors as well.
This is accomplished via a function named trigger_error()
, which allows you
to raise any of the three error types reserved for users: E_USER_NOTICE
,
E_USER_WARNING
and E_USER_ERROR
. When these errors are triggered,
PHP's built-in handler will automatically wake up to handle them.
<?php
// function to test a number
// generates E_USER_WARNING if number is a float
// generates E_USER_ERROR is number is negative
function testNumber($num) {
// float
// trigger a warning
if (is_float($num)) {
trigger_error("Number $num is not an integer", E_USER_WARNING);
}
// negative
// trigger a fatal error
if ($num < 0) {
trigger_error("Number $num is negative", E_USER_ERROR);
}
}
// test the function with different values
testNumber(100);
testNumber(5.6);
testNumber(-8);
?>
If you'd like to have a custom error handler to handle your custom errors... well, you're
just hard to please, aren't you? Take a look at this next example, which rewrites the
previous script to use a user-defined error handler:
<?php
// function to test a number
// generates E_USER_WARNING if number is a float
// generates E_USER_ERROR is number is negative
function testNumber($num) {
// float
// trigger a warning
if (is_float($num)) {
trigger_error("Number $num is not an integer", E_USER_WARNING);
}
// negative
// trigger a fatal error
if ($num < 0) {
trigger_error("Number $num is negative", E_USER_ERROR);
}
}
// custom error handler
function myErrorHandler($type, $msg, $file, $line, $context) {
switch ($type) {
// warnings
case E_USER_WARNING:
// report error
print "Non-fatal error on line $line of $file: $msg <br />";
break;
// fatals
case E_USER_ERROR:
// report error and die()
die("Fatal error on line $line of $file: $msg <br />");
break;
// notices
default:
// do nothing
break;
}
}
// set the name of the custom handler
set_error_handler('myErrorHandler');
// test the function with different values
testNumber(100);
testNumber(5.6);
testNumber(-8);
?>
Note that it is the responsibility of the custom handler to die()
in the
event of user-generated fatal errors - PHP will not do this automatically.
You can use the same method to deal with exceptions too. Scroll on down, and
let me show you how.
Catching Up
If you're using PHP 5, you also have an alternative to the techniques discussed
so far in the new Exception model (exception is Geek for error). Exceptions are new
to PHP (although they've been in languages like Java and Python for ages) and they're
stirring up a great deal of excitement.
In the exception-based approach, program code is wrapped in a try()
block,
and exceptions generated by it are "caught" and resolved by a catch()
block.
Multiple catch()
blocks are possible, each one dealing with a different
error type; this allows developers to trap different types of errors and execute
appropriate exception-handling.
Here's what a typical try-catch()
block looks like:
try {
execute this block
}
catch (exception type 1) {
execute this block to resolve exception type 1
}
catch (exception type 2) {
execute this block to resolve exception type 2
}
... and so on ...
When PHP encounters code wrapped within a try-catch()
block, it first
attempts to execute the code within the try()
block. If this code is
processed without any exceptions being generated, control transfers to the lines
following the try-catch()
block. However, if an exception is generated
while running the code within the try()
block, PHP stops execution of
the block at that point and begins checking each catch()
block to see
if there is a handler for the exception. If a handler is found, the code within the
appropriate catch()
block is executed; if not, a fatal error is generated.
It is even possible to handle that fatal error in a nice way using exceptions; see
http://www.php.net/set-exception-handler for more on this.
The exceptions themselves are generated via PHP's throw
statement. The
throw
statement needs to be passed a descriptive message, and an optional
error code. When the exception is raised, this description and code will be made
available to the exception handler.
Wanna see how this works? Here's an example:
<?php
// PHP 5
error_reporting(0);
// try this code
try {
$file = 'somefile.txt';
// open file
if (!$fh = fopen($file, 'r')) {
throw new Exception('Could not open file!');
}
// read file contents
if (!$data = fread($fh, filesize($file))) {
throw new Exception('Could not read file!');
}
// close file
fclose($fh);
// print file contents
echo $data;
}
// catch errors if any
catch (Exception $e) {
print 'Something bad just happened...';
}
?>
If the file doesn't exist or is unreadable, the throw
statement will
generate an exception (basically, an instance of PHP's built-in Exception
object) and pass it a message describing the error. When such an exception is generated,
control passes to the first catch()
block. If the catch()
block
can handle the exception type, the code within the catch()
block is executed.
If the first catch()
block cannot handle the generated exception, control
passes to the next one.
Don't worry too much about "exception types" at this point - all will be explained shortly.
For the moment, all you need to know is that the generic catch()
block above
will catch all exceptions, regardless of type.
Now, there's one problem with the previous listing. Although the catch()
block will trap the exception and print a message, it can't display the descriptive
message sent by the throw
statement with the exception. To access this
message, as well as a couple of other interesting pieces of information, it is necessary
to use some of the Exception
object's built-in methods. Take a look at this
revision of the previous script, which illustrates:
<?php
// PHP 5
error_reporting(0);
// try this code
try {
$file = 'somefile.txt';
// open file
if (!$fh = fopen($file, 'r')) {
throw new Exception('Could not open file!', 12);
}
// read file contents
if (!$data = fread($fh, filesize($file))) {
throw new Exception('Could not read file!', 9);
}
// close file
fclose($fh);
// print file contents
echo $data;
}
// catch errors if any
catch (Exception $e) {
print '<h2>Exception</h2>';
print 'Error message: ' . $e->getMessage() . '<br />';
print 'Error code: ' . $e->getCode() . '<br />';
print 'File and line: ' . $e->getFile() . '(' . $e->getLine() . ')<br />';
print 'Trace: ' . $e->getTraceAsString() . '<br />';
}
?>
When you run this script, you'll see that the message generated by the exception handler contains:
- the descriptive data sent by
throw
, - an error code (also sent by
throw
), - the file name and line number where the exception occurred, and
- a stack trace indicating the exception's progress through the class hierarchy, if there is one.
This data is generated by calling the
Exception
object's
getMessage()
,
getCode()
,
getFile()
,
getLine()
and
getTraceAsString()
methods respectively inside
the
catch()
block.
Adding Some Class
You can handle different exceptions in different ways, by sub-classing the generic
Exception
object and using more than one catch()
block. The
following example is a simple illustration of this:
<?php
// PHP 5
// sub-class the Exception class
class NegativeNumException extends Exception {}
class OutOfRangeException extends Exception {}
class FloatException extends Exception {}
// function to test a number
function testNumber($num) {
// float
// trigger an exception
if (is_float($num)) {
throw new FloatException($num);
}
// negative
// trigger an exception
if ($num < 0) {
throw new NegativeNumException($num);
}
// out of range
// trigger an exception
if ($num > 1000 || $num < 100) {
throw new OutOfRangeException($num);
}
}
// try this code
try {
testNumber(-19);
}
// catch errors, if any
catch (NegativeNumException $e) {
print 'A negative number was provided ('.$e->getMessage().'). Please provide a positive integer between 100 and 1000.<br />';
}
catch (OutOfRangeException $e) {
print 'The number provided is out of range ('.$e->getMessage().'). Please provide a positive integer between 100 and 1000.<br />';
}
catch (FloatException $e) {
print 'The number provided is not an integer ('.$e->getMessage().'). Please provide a positive integer between 100 and 1000.<br />';
}
catch (Exception $e) {
print 'Error message: ' . $e->getMessage() . '<br />';
print 'Error code: ' . $e->getCode() . '<br />';
print 'File and line: ' . $e->getFile() . '(' . $e->getLine() . ')<br />';
print 'Trace: ' . $e->getTraceAsString() . '<br />';
}
?>
In this case, I've created three new Exception
sub-classes from the base
object, one for each possible error. Next, I've set up catch()
blocks for
each exception type, and written exception-handling code that is specific to each type.
Depending on which exception occurs (you can generate different ones by sending the
testNumber()
function different values), the appropriate catch()
block will be invoked and a different error message will be printed.
Note that because PHP will always use the first catch()
block that matches
the exception type, and because the generic Exception
class matches all
exceptions, the catch()
blocks must be arranged in the order of most
specific first. This has been done in the example above, where the generic
catch()
block appears last on the list.
Here's another example, this one illustrating a more useful application - using the
exception model in a user authentication class to provide easy-to-understand error
handling. Take a look:
<?php
// PHP 5
// class definition
class userAuth {
// define properties
private $username;
private $passwd;
private $passwdFile;
// constructor
// must be passed username and non-encrypted password
public function __construct($username, $password) {
$this->username = $username;
$this->passwd = $password;
}
// set .htaccess-style file to check for passwords
public function setPasswdFile($file) {
$this->passwdFile = $file;
}
// perform password verification
public function authenticateUser() {
// check that the file exists
if (!file_exists($this->passwdFile)) {
throw new FileException("Password file cannot be found: " . $this->passwdFile);
}
// check that the file is readable
if (!is_readable($this->passwdFile)) {
throw new FileException("Unable to read password file: ". $this->passwdFile);
}
// read file
$data = file($this->passwdFile);
// iterate through file
foreach ($data as $line) {
$arr = explode(":", $line);
// if username matches, test password
if ($arr[0] == $this->username) {
// get salt and crypt(), assuming encryption
$salt = substr($arr[1], 0, 2);
// if match, user/pass combination is correct
if ($arr[1] == crypt($this->passwd, $salt)) {
echo "User was authenticated";
// do some other stuff
}
// otherwise return exception
else {
throw new AuthException("Incorrect password");
break;
}
}
else {
// could not find a username match
// return exception
throw new AuthException("No such user");
}
}
}
// end class definition
}
// subclass exceptions
class FileException extends Exception {};
class AuthException extends Exception {};
// try the code
try {
// create instance
$ua = new userAuth("joe", "secret");
// set password file
$ua->setPasswdFile("password.txt");
// perform authentication
$ua->authenticateUser();
}
// catch authentication failures, if any
catch (FileException $e) {
// print file errors
print "A file error occurred. ".$e->getMessage();
}
catch (AuthException $e) {
// an authentication error occurred
print "An authentication error occurred. ".$e->getMessage();
// more normally, redirect to new page on auth errors, e.g.
// header ('Location: login_fail.php');
}
catch (Exception $e) {
print "An unknown error occurred";
}
?>
Here, depending on the type of error, either a FileException()
or an
AuthException()
will be generated - and handled by the corresponding
catch()
block. Notice how easy the exception handling framework is to read
and extend. It's precisely this ease of use and extensibility that helps the new PHP 5
model score over the earlier, more primitive techniques of handling application errors.
Well, that's about it for the moment. Come back soon, for more PHP 101!