exceptions.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<chapter xml:id="language.exceptions" xmlns="http://docbook.org/ns/docbook">
 <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>

  <sect1 annotations="chunk:false" xml:id="language.exceptions.catch">
   <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>

  <sect1 annotations="chunk:false" xml:id="language.exceptions.finally">
   <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>
  </sect1>

 <sect1 annotations="chunk:false" xml:id="language.exceptions.exception-handler">
  <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>

 <sect1 annotations="chunk:false" xml:id="language.exceptions.notes">
   &reftitle.notes;

   <note>
    <para>
     Internal PHP functions mainly use
     <link linkend="ini.error-reporting">Error reporting</link>, only modern
     <link linkend="language.oop5">Object-oriented</link>
     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>
    <example>
     <title>Converting error reporting to exceptions</title>
     <programlisting role="php">
<![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>

  <sect1 annotations="chunk:false" xml:id="language.exceptions.examples">
   &reftitle.examples;

   <example>
    <title>Throwing an Exception</title>
    <programlisting role="php">
<![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;
    <screen>
<![CDATA[
0.2
Caught exception: Division by zero.
Hello World
]]>
    </screen>
   </example>
   <example>
    <title>Exception handling with a &finally; block</title>
    <programlisting role="php">
<![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;
    <screen>
<![CDATA[
0.2
First finally.
Caught exception: Division by zero.
Second finally.
Hello World
]]>
    </screen>
   </example>
   <example>
    <title>Interaction between the &finally; block and &return;</title>
    <programlisting role="php">
<![CDATA[
<?php

function test() {
    try {
        throw new Exception('foo');
    } catch (Exception $e) {
        return 'catch';
    } finally {
        return 'finally';
    }
}

echo test();
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
finally
]]>
    </screen>
   </example>
   <example>
    <title>Nested Exception</title>
    <programlisting role="php">
<![CDATA[
<?php

class MyException extends Exception { }

class Test {
    public function testing() {
        try {
            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;
    <screen>
<![CDATA[
string(4) "foo!"
]]>
    </screen>
   </example>
   <example>
    <title>Multi catch exception handling</title>
    <programlisting role="php">
<![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;
    <screen>
<![CDATA[
string(11) "MyException"
]]>
    </screen>
   </example>
   <example>
    <title>Omitting the caught variable</title>
    <para>Only permitted in PHP 8.0.0 and later.</para>
    <programlisting role="php">
<![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;
    <screen>
<![CDATA[
A SpecificException was thrown, but we don't care about the details.
]]>
    </screen>
   </example>
   <example>
    <title>Throw as an expression</title>
    <para>Only permitted in PHP 8.0.0 and later.</para>
    <programlisting role="php">
<![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;
    <screen>
<![CDATA[
It did not work
]]>
    </screen>
   </example>
  </sect1>

 <sect1 xml:id="language.exceptions.extending">
  <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>
  <example>
   <title>The Built in Exception class</title>
   <programlisting role="php">
    <![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>
  <example>
   <title>Extending the Exception class</title>
   <programlisting role="php">
    <![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>
 
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