doc-en/language/exceptions.xml at master · php/doc-en

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<?xml version="1.0" encoding="utf-8"?>

<title>Exceptions</title>

<para>

PHP has an exception model similar to that of other programming

languages. An exception can be &throw;n, and caught ("&catch;ed") within

PHP. Code may be surrounded in a &try; block, to facilitate the catching

of potential exceptions. Each &try; must have at least one corresponding

&catch; or &finally; block.

</para>

<para>

If an exception is thrown and its current function scope has no &catch;

block, the exception will "bubble up" the call stack to the calling

function until it finds a matching &catch; block. All &finally; blocks it encounters

along the way will be executed. If the call stack is unwound all the way to the

global scope without encountering a matching &catch; block, the program will

terminate with a fatal error unless a global exception handler has been set.

</para>

<para>

The thrown object must be an &instanceof; <interfacename>Throwable</interfacename>.

Trying to throw an object that is not will result in a PHP Fatal Error.

</para>

<para>

As of PHP 8.0.0, the &throw; keyword is an expression and may be used in any expression

context. In prior versions it was a statement and was required to be on its own line.

</para>

<title><literal>catch</literal></title>

<para>

A &catch; block defines how to respond to a thrown exception. A &catch;

block defines one or more types of exception or error it can handle, and

optionally a variable to which to assign the exception. (The variable was

required prior to PHP 8.0.0.) The first &catch; block a thrown exception

or error encounters that matches the type of the thrown object will handle

the object.

</para>

<para>

Multiple &catch; blocks can be used to catch different classes of

exceptions. Normal execution (when no exception is thrown within the &try;

block) will continue after that last &catch; block defined in sequence.

Exceptions can be &throw;n (or re-thrown) within a &catch; block. If not,

execution will continue after the &catch; block that was triggered.

</para>

<para>

When an exception is thrown, code following the statement will not be

executed, and PHP will attempt to find the first matching &catch; block.

If an exception is not caught, a PHP Fatal Error will be issued with an

"<literal>Uncaught Exception ...</literal>" message, unless a handler has

been defined with <function>set_exception_handler</function>.

</para>

<para>

As of PHP 7.1.0, a &catch; block may specify multiple exceptions

using the pipe (<literal>|</literal>) character. This is useful for when

different exceptions from different class hierarchies are handled the

same.

</para>

<para>

As of PHP 8.0.0, the variable name for a caught exception is optional.

If not specified, the &catch; block will still execute but will not

have access to the thrown object.

</para>

</sect1>

<title><literal>finally</literal></title>

<para>

A &finally; block may also be specified after or

instead of &catch; blocks. Code within the &finally; block will always be

executed after the &try; and &catch; blocks, regardless of whether an

exception has been thrown, and before normal execution resumes.

</para>

<para>

One notable interaction is between the &finally; block and a &return; statement.

If a &return; statement is encountered inside either the &try; or the &catch; blocks,

the &finally; block will still be executed. Moreover, the &return; statement is

evaluated when encountered, but the result will be returned after the &finally; block

is executed. Additionally, if the &finally; block also contains a &return; statement,

the value from the &finally; block is returned.

</para>

<para>

Another notable interaction is between an exception thrown from within a &try; block,

and an exception thrown from within a &finally; block. If both blocks throw an exception,

then the exception thrown from the &finally; block will be the one that is propagated,

and the exception thrown from the &try; block will be used as its previous exception.

</para>

</sect1>

<title>Global exception handler</title>

<para>

If an exception is allowed to bubble up to the global scope, it may be caught

by a global exception handler if set. The <function>set_exception_handler</function>

function can set a function that will be called in place of a &catch; block if no

other block is invoked. The effect is essentially the same as if the entire program

were wrapped in a &try;-&catch; block with that function as the &catch;.

</para>

</sect1>

&reftitle.notes;

<note>

<para>

Internal PHP functions mainly use

extensions use exceptions. However, errors can be easily translated to

exceptions with <link linkend="class.errorexception">ErrorException</link>.

This technique only works with non-fatal errors, however.

</para>

<title>Converting error reporting to exceptions</title>

<![CDATA[

<?php

function exceptions_error_handler($severity, $message, $filename, $lineno) {

throw new ErrorException($message, 0, $severity, $filename, $lineno);

}

set_error_handler('exceptions_error_handler');

]]>

</programlisting>

</example>

</note>

<tip>

<para>

The <link linkend="intro.spl">Standard PHP Library (SPL)</link> provides

a good number of <link linkend="spl.exceptions">built-in

exceptions</link>.

</para>

</tip>

</sect1>

&reftitle.examples;

<title>Throwing an Exception</title>

<![CDATA[

<?php

function inverse($x) {

if (!$x) {

throw new Exception('Division by zero.');

}

return 1/$x;

}

try {

echo inverse(5) . "\n";

echo inverse(0) . "\n";

} catch (Exception $e) {

echo 'Caught exception: ', $e->getMessage(), "\n";

}

// Continue execution

echo "Hello World\n";

]]>

</programlisting>

&example.outputs;

<![CDATA[

0.2

Caught exception: Division by zero.

Hello World

]]>

</screen>

</example>

<title>Exception handling with a &finally; block</title>

<![CDATA[

<?php

function inverse($x) {

if (!$x) {

throw new Exception('Division by zero.');

}

return 1/$x;

}

try {

echo inverse(5) . "\n";

} catch (Exception $e) {

echo 'Caught exception: ', $e->getMessage(), "\n";

} finally {

echo "First finally.\n";

}

try {

echo inverse(0) . "\n";

} catch (Exception $e) {

echo 'Caught exception: ', $e->getMessage(), "\n";

} finally {

echo "Second finally.\n";

}

// Continue execution

echo "Hello World\n";

]]>

</programlisting>

&example.outputs;

<![CDATA[

0.2

First finally.

Caught exception: Division by zero.

Second finally.

Hello World

]]>

</screen>

</example>

<title>Interaction between the &finally; block and &return;</title>

<![CDATA[

<?php

function test() {

try {

throw new Exception('foo');

} catch (Exception $e) {

return 'catch';

} finally {

return 'finally';

}

echo test();

]]>

</programlisting>

&example.outputs;

<![CDATA[

finally

]]>

</screen>

</example>

<title>Nested Exception</title>

<![CDATA[

<?php

class MyException extends Exception { }

class Test {

public function testing() {

try {

throw new MyException('foo!');

} catch (MyException $e) {

// rethrow it

throw $e;

}

} catch (Exception $e) {

var_dump($e->getMessage());

}

$foo = new Test;

$foo->testing();

]]>

</programlisting>

&example.outputs;

<![CDATA[

string(4) "foo!"

]]>

</screen>

</example>

<title>Multi catch exception handling</title>

<![CDATA[

<?php

class MyException extends Exception { }

class MyOtherException extends Exception { }

class Test {

public function testing() {

try {

throw new MyException();

} catch (MyException | MyOtherException $e) {

var_dump(get_class($e));

}

$foo = new Test;

$foo->testing();

]]>

</programlisting>

&example.outputs;

<![CDATA[

string(11) "MyException"

]]>

</screen>

</example>

<title>Omitting the caught variable</title>

<para>Only permitted in PHP 8.0.0 and later.</para>

<![CDATA[

<?php

class SpecificException extends Exception {}

function test() {

throw new SpecificException('Oopsie');

}

try {

test();

} catch (SpecificException) {

print "A SpecificException was thrown, but we don't care about the details.";

}

]]>

</programlisting>

&example.outputs;

<![CDATA[

A SpecificException was thrown, but we don't care about the details.

]]>

</screen>

</example>

<title>Throw as an expression</title>

<para>Only permitted in PHP 8.0.0 and later.</para>

<![CDATA[

<?php

function test() {

do_something_risky() or throw new Exception('It did not work');

}

function do_something_risky() {

return false; // Simulate failure

}

try {

test();

} catch (Exception $e) {

print $e->getMessage();

}

]]>

</programlisting>

&example.outputs;

<![CDATA[

It did not work

]]>

</screen>

</example>

<title>Exception in try and in finally</title>

<![CDATA[

<?php

try {

throw new Exception(message: 'Third', previous: new Exception('Fourth'));

} finally {

throw new Exception(message: 'First', previous: new Exception('Second'));

}

} catch (Exception $e) {

var_dump(

$e->getMessage(),

$e->getPrevious()->getMessage(),

$e->getPrevious()->getPrevious()->getMessage(),

$e->getPrevious()->getPrevious()->getPrevious()->getMessage(),

);

}

]]>

</programlisting>

&example.outputs;

<![CDATA[

string(5) "First"

string(6) "Second"

string(5) "Third"

string(6) "Fourth"

]]>

</screen>

</example>

</sect1>

<title>Extending Exceptions</title>

<para>

A User defined Exception class can be defined by extending the built-in

Exception class. The members and properties below, show what is accessible

within the child class that derives from the built-in Exception class.

</para>

<title>The Built in Exception class</title>

<![CDATA[

<?php

class Exception implements Throwable

{

protected $message = 'Unknown exception'; // exception message

private $string; // __toString cache

protected $code = 0; // user defined exception code

protected $file; // source filename of exception

protected $line; // source line of exception

private $trace; // backtrace

private $previous; // previous exception if nested exception

public function __construct($message = '', $code = 0, ?Throwable $previous = null);

final private function __clone(); // Inhibits cloning of exceptions.

final public function getMessage(); // message of exception

final public function getCode(); // code of exception

final public function getFile(); // source filename

final public function getLine(); // source line

final public function getTrace(); // an array of the backtrace()

final public function getPrevious(); // previous exception

final public function getTraceAsString(); // formatted string of trace

// Overrideable

public function __toString(); // formatted string for display

}

]]>

</programlisting>

</example>

<para>

If a class extends the built-in Exception class and re-defines the <link

linkend="language.oop5.decon">constructor</link>, it is highly recommended

that it also call <link

linkend="language.oop5.paamayim-nekudotayim">parent::__construct()</link>

to ensure all available data has been properly assigned. The <link

linkend="language.oop5.magic">__toString()</link> method can be overridden

to provide a custom output when the object is presented as a string.

</para>

<note>

<para>

Exceptions cannot be cloned. Attempting to <link

linkend="language.oop5.cloning">clone</link> an Exception will result in a

fatal <constant>E_ERROR</constant> error.

</para>

</note>

<title>Extending the Exception class</title>

<![CDATA[

<?php

/**

* Define a custom exception class

class MyException extends Exception

{

// Redefine the exception so message isn't optional

public function __construct($message, $code = 0, ?Throwable $previous = null) {

// some code

// make sure everything is assigned properly

parent::__construct($message, $code, $previous);

}

// custom string representation of object

public function __toString() {

return __CLASS__ . ": [{$this->code}]: {$this->message}\n";

}

public function customFunction() {

echo "A custom function for this type of exception\n";

}

/**

* Create a class to test the exception

class TestException

{

public $var;

const THROW_NONE = 0;

const THROW_CUSTOM = 1;

const THROW_DEFAULT = 2;

function __construct($avalue = self::THROW_NONE) {

switch ($avalue) {

case self::THROW_CUSTOM:

// throw custom exception

throw new MyException('1 is an invalid parameter', 5);

break;

case self::THROW_DEFAULT:

// throw default one.

throw new Exception('2 is not allowed as a parameter', 6);

break;

default:

// No exception, object will be created.

$this->var = $avalue;

break;

}

// Example 1

try {

$o = new TestException(TestException::THROW_CUSTOM);

} catch (MyException $e) { // Will be caught

echo "Caught my exception\n", $e;

$e->customFunction();

} catch (Exception $e) { // Skipped

echo "Caught Default Exception\n", $e;

}

// Continue execution

var_dump($o); // Null

echo "\n\n";

// Example 2

try {

$o = new TestException(TestException::THROW_DEFAULT);

} catch (MyException $e) { // Doesn't match this type

echo "Caught my exception\n", $e;

$e->customFunction();

} catch (Exception $e) { // Will be caught

echo "Caught Default Exception\n", $e;

}

// Continue execution

var_dump($o); // Null

echo "\n\n";

// Example 3

try {

$o = new TestException(TestException::THROW_CUSTOM);

} catch (Exception $e) { // Will be caught

echo "Default Exception caught\n", $e;

}

// Continue execution

var_dump($o); // Null

echo "\n\n";

// Example 4

try {

$o = new TestException();

} catch (Exception $e) { // Skipped, no exception

echo "Default Exception caught\n", $e;

}

// Continue execution

var_dump($o); // TestException

echo "\n\n";

]]>

</programlisting>

</example>

</sect1>

</chapter>

<!-- Keep this comment at the end of the file

Local variables:

mode: sgml

sgml-omittag:t

sgml-shorttag:t

sgml-minimize-attributes:nil

sgml-always-quote-attributes:t

sgml-indent-step:1

sgml-indent-data:t

indent-tabs-mode:nil

sgml-parent-document:nil

sgml-default-dtd-file:"~/.phpdoc/manual.ced"

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sgml-local-ecat-files:nil

End:

vim600: syn=xml fen fdm=syntax fdl=2 si

vim: et tw=78 syn=sgml

vi: ts=1 sw=1

-->

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