Introduction to php exception and error management

Types of errors
Compile-time errors Errors detected by the parser while it is compiling
a script. Cannot be trapped from within the script itself.
Fatal errors Errors that halt the execution of a script. Cannot be trapped.
Recoverable errors Errors that represent significant failures, but can still be
handled in a safe way.
Warnings Recoverable errors that indicate a run-time fault. Do not halt
the execution of the script.
Notices Indicate that an error condition occurred, but is not
necessarily significant. Do not halt the execution of the script.

Error Reporting
• By default, PHP reports any errors it encounters to the script’s
output unless you control it as follows
1. Configuring Php.ini
2. Handling Errors by setting set_error_handler()

– Several configuration directives in the php.ini file allow you to fine tune
how—and which—errors are reported.
– error_reporting,
– display_errors
– log_errors.
You can find these in php.ini file

• error_reporting
– determines which errors are reported by PHP
– Eg: error_reporting=E_ALL & ~E_NOTICE
• display_errors
– if turned on, errors are outputted to the script’s output; this is not
desirable in a production environment, as everyone will be able to see
your scripts’ errors. Eg: display_errors = ON
• log_errors,
– which causes errors to be written to your web server’s error log, so that
only developers can utilise it and end users will not be informed of same
– Eg:log_errors,=ON

2. Handling Errors
• The set_error_handler() function sets a user-defined function to
handle errors.
• Eg: set_error_handler(error_function,error_types)
<?php
//error handler function
function customError($errno, $errstr, $errfile,
$errline)
{
echo "<b>Custom error:</b> [$errno] $errstr<br
/>";
echo " Error on line $errline in $errfile<br />";
echo "Ending Script";
die();
}
//set error handler
set_error_handler("customError");
$test=2;
//trigger error
if ($test>1)
{
trigger_error("A custom error has been
triggered");
}

Exception Handling in PHP
• Exceptions provide an error control mechanism that is more fine-
grained than traditional PHP fault handling.There are several key
differences between “regular” PHP errors and exceptions:
• Exceptions are objects, created (or “thrown”) when an error occurs
• Exceptions can be handled at different points in a script’s execution, and
different types of exceptions can be handled by separate portions of a
script’s code
• All unhandled exceptions are fatal
• Exceptions can be thrown from the __construct method on failure
• Exceptions change the flow of the application

The Basic Exception Class
• As we mentioned in the previous paragraph, exceptions are objects
that must be direct or indirect (for example through inheritance)
instances of the Exception base class.
• The latter is built into PHP itself, and is declared as follows:

class Exception {
// The error message associated with this exception
protected $message = ’Unknown Exception’;
// The error code associated with this exception
protected $code = 0;
// The pathname of the file where the exception occurred
protected $file;
// The line of the file where the exception occurred
protected $line;
// Constructor
function __construct ($message = null, $code =
0);
// Returns the message
final function getMessage();
// Returns the error code
final function getCode();
// Returns the file name
final function getFile();
// Returns the file line
final function getLine();
// Returns an execution backtrace as an array
final function getTrace();
// Returns a backtrace as a string
final function getTraceAsString();
// Returns a string representation of the
exception
function __toString();
}

• Almost all of the properties of an Exception are automatically
filled in for you by the interpreter—generally speaking, you only
need to provide a message and a code, and all the remaining
information will be taken care of for you.
• Since Exception is a normal (if built-in) class, you can extend it
and effectively create your own exceptions, thus providing your
error handlers with any additional information that your
application requires.

Throwing Exceptions
• Exceptions are usually created and thrown when an error occurs by using
the throw construct:
if ($error) {
throw new Exception ("This is my error");
}
• Exceptions then “bubble up” until they are either handled by the script or
cause a fatal exception
• The handling of exceptions is performed using a try...catch block:

Example
try {
if ($error) {
throw new Exception ("This is my error");
}
} catch (Exception $e) {
echo $e->getMessage();
}
• In the example above, any exception that is thrown inside the try{} block is
going to be caught and passed on the code inside the catch{} block, where it
can be handled

Lazy Loading
• Prior to PHP 5, if you try to create an object on an undefined class(perhaps
that class is written on another file which you forgot to include) would
cause a fatal error.
• This meant that you needed to include all of the class files that you might
need, rather than loading them as they were needed
• To solve this problem, PHP 5 features an “autoload” facility that makes it
possible to implement “lazy loading”, or loading of classes on-demand only
• When we try to create an object of undefined class PHP will try to call the
__autoload() global magic function so that the script may be given an
opportunity to load it.

__autoload() - Example
function __autoload($class)
{
// Require PEAR-compatible classes
require_once(“$class.php”)
}
$obj = new SomeClass();

{
}
You tried to create an object of
SomeClass. But you forgot that someClass
is defined in another page called
SomeClass.php

{
}
When you tried to do so, PHP will
automatically calls __autoload() which
will have an argument ie the class name
for which object we tried to create

{
}
We are including the file called
“someClass.php”
The advantage of lazy loading is that we can include files upon its requirement only
rather than including all the files unnecessarily

Questions?
“A good question deserve a good grade…”

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