Error Handling in PHP

Jussi Pohjolainen

TAMK University of Applied Sciences » ICT

PHP Error Handling

• PHP has three major levels of errors:
• E_NOTICE
• E_WARNING
• E_ERROR

E_NOTICE

• E_NOTICE: you may do something that you don't want to do.
• Well implemented app doesn't have any E_NOTICE - "errors".
• Example:
  • $var++; // $var is not initialized
• E_NOTICE is off by default.

E_WARNING, E_ERROR and others

• E_WARNING: nonfatal runtime error.
• E_ERROR: unrecoverable error.
• Other errors include:
  • E_PARSE
  • E_COMPILE_ERROR
  • E_COMPILE_WARNING
  • E_CORE_ERROR
  • E_CORE_WARNING

Setting Error Reporting Levels

• Modify php.ini file:
• error_reporting = E_ALL
• error_reporting = E_ALL ~ E_NOTICE // Default
• Or set in script: error_reporting

Handling errors

• You can display, log, ignore or act on errors.
• Display: php.ini: display_errors = On;
• Log: php.ini: log_error = On; error_log = /path/to/filename
• Ignore: $fp = @fopen(...)
• Acting on errors: Custom error handler: set_error_handler(..)

Method errors

• What should method do, when an error occures?
• Return false? Return null? Return -1? Return string?
• How do you know when method return value is valid and when return value is an error?
• Very confusing.

Exceptions

• Before PHP 5 exception handling was not possible
• PHP5 exception handling is very similar to Java
• Mechanism that is designed to handle errors

Exceptions

• In PHP5, exception is an object
• PHP5 has built-in Exception-class that is designed specifically for exceptions
• Creating exception: create object from exception class and throw it.

Example: throwing exception

function divide ($a, $b)
{
    if( ! (is_int($a)  and is_int($b)) )
    {
        throw new Exception();
    }
    if( $b === 0 )
    {
        throw new Exception();
    }
    
    return ( $a / $b );
}

Example: catching exception

try 
{
    $divide = divide( 3, 3 );
}
catch ( Exception $e )
{
    print "Problem";    
}

Creating your own Exception-classes

class AritmeticException extends Exception { }
class NumberFormatException extends Exception { }

function divide ($a, $b)
{
    if( ! (is_int($a)  and  is_int($b)) )
    {
        throw new NumberFormatException();
    }
    if( $b === 0 )
    {
        throw new AritmeticException();
    }
    
    return ( $a / $b );
}

Example: catching your own exception

try 
{
    $divide = divide( 3, 3 );
}
catch ( NumberFormatException $e )
{
    print "Please give integer-values";    
}
catch ( ArithmeticException $e )
{
    print "You cannot divide with 0";    
}

Exception class

class Exception
{
    protected $message = 'Unknown exception';   // exception message
    protected $code = 0;                        // user defined exception code
    protected $file;                            // source filename of exception
    protected $line;                            // source line of exception
    function __construct($message = null, $code = 0);
    final function getMessage();                // message of exception 
    final function getCode();                   // code of exception
    final function getFile();                   // source filename
    final function getLine();                   // source line
    final function getTrace();                  // an array of the backtrace()
    final function getTraceAsString();          // formated string of trace
    function __toString();                      // formated string for display
}

Making a better version of own Exceptions

class AritmeticException extends Exception 
{ 
    function __construct( $msg )
    {
        parent::__construct( $msg );
    }
}

function divide ($a, $b)
{
    if( ! (is_int($a)  and  is_int($b)) )
    {
        throw new NumberFormatException();
    }
    if( $b === 0 )
    {
        throw new AritmeticException("You cannot divide with zero");
    }
    
    return ( $a / $b );
}