CSCI 215 Web Programming II

Debugging & Error Handling

2

Error TypesParse (syntax) errors

◦Occur when the scripting engine fails to recognize code

◦Example: Incorrectly spelled or mistyped wordsRun-time errors

◦Occur when the engine encounters a problem while program is running

◦Example: Division by zeroLogic errors

◦Flaws in program design that prevent the program from running as intended

◦Example: Infinite loop

PHP Programming 3

Parse Error

11 <?php

12 for ($count = 10; $count >= 0; --$count)

13 if ($count == 0)

14 echo "<p>We have liftoff!</p>";

15 else

16 echo "<p>Liftoff in $count seconds.</p>";

17 }

18 ?>

PHP Programming 4

Parse Error

Run-Time ErrorsE_NOTICE Indicate that the script encountered something

that could indicate an error, but could also happen in the normal course of running a script.

E_USER_NOTICE User-generated notice message, generated by trigger_error().

E_WARNING Run-time warnings (non-fatal errors). Execution of the script is not halted.

E_USER_WARNING User-generated warning message, generated by trigger_error().

E_COMPILE_WARNING Fatal compile-time errors.

E_CORE_WARNING Warnings that occur during PHP's initial startup.

E_ERROR Fatal run-time errors. These indicate errors that can not be recovered from, such as a memory allocation problem. Execution of the script is halted.

E_USER_ERROR User-generated error message, generated by trigger_error().

E_COMPILE_ERROR Fatal compile-time errors.

E_CORE_ERROR Fatal errors that occur during PHP's initial startup.

E_PARSE Compile-time parse (syntax) errors.

notices

warnings

fatalerrors

PHP Programming 6

NoticesRaised for potential run-time

errors that do not prevent a script from executing

Examples◦use of an undefined variable◦defining a string without quotes

Notices<?php

error_reporting(E_ALL);

echo '<p>Error 1: Undefined variable:</p>';

echo $undefined_var;

echo '<p>Error 2: Unquoted string:</p>';

$some_var = tryetrhrtdf;

?>

http://ned.highline.edu/~tostrander/215/error_handling/error1.php

Notices

PHP Programming 9

WarningsDo not prevent a script from

executingIndicate that something clearly

wrong has happened and action should be taken.

Examples:◦File not found◦Database not available◦Missing function arguments

PHP Programming 10

Warningsfunction beginCountdown($time)

{

if (!isset($time))

$time = 10;

for ($count = $time; $count >= 0; —$count) {

if ($count == 0)

echo '<p>We have liftoff!</p>';

else

echo "<p>Liftoff in $count

seconds.</p>";

}

}

beginCountdown();

PHP Programming 11

Warnings

Warnings<?

echo '<p>Error 1: file not available:</p>';

$fp = fopen('file_does_not_exist.dat','r');

echo '<p>Error 2: db not available:</p>';

$results = mysql_query('SOME QUERY');

?>

http://ned.highline.edu/~tostrander/215/error_handling/error2.php

Warnings

PHP Programming 14

Fatal ErrorsRaised when a run-time error

prevents a script from executingSomething so terrible has

happened during execution of your script that processing cannot continue.

Examples:◦Parse error◦Calling an undefined function

Fatal Errors<?

error_reporting(E_ALL);

echo '<p>Error 1: Undefined function:</p>';

$fp = non_existing_function('an_arg');

echo 'Code never gets here!!!';

?>

http://ned.highline.edu/~tostrander/215/error_handling/error3.php

Raising ErrorsIt is possible to force a PHP error

at any point in your script.trigger_error($msg, $type);

Example:

if (!$name) {

trigger_error('No name entered', E_USER_ERROR);

}

…

PHP Programming 17

trigger_error()trigger_error() accepts two

arguments:◦A custom error message◦The error reporting level

E_USER_ERROR E_USER_WARNING E_USER_NOTICE

PHP Programming 18

if (isset($_GET['height']) &&

isset($_GET['weight'])) {

if (!is_numeric($_GET['weight'])

|| !is_numeric($_GET['height'])) {

trigger_error(“User did not enter numeric

values”, E_USER_ERROR);

}

} else {

trigger_error(“Values not entered”,

E_USER_ERROR);

}

$bodyMass = $_GET['weight'] / ($_GET['height']

* $_GET['height']) * 703;

print "Your body mass index is $bodyMass";

PHP Programming 19

PHP Programming 20

php.ini Directivesdisplay_errors

◦prints script error messages ◦default value of “On”

error_reporting ◦determines which types of error

messages PHP should generate◦by default, assigned a value of

“E_ALL”

Set Error Reporting Level

error_reporting($level)Used to control which errors are

displayed, and which are ignored. Only lasts for the duration of your

script

ini_set('display_errors', 1);

// turn all error reporting ON// error_reporting(E_ALL);

// Report all errors except E_NOTICE// error_reporting(E_ALL ^ E_NOTICE);

// Turn off all error reporting error_reporting(0);

// generate errorsecho '<p>Error 1: Use of an undefined variable.</p>';echo $undefined_var;echo '<p>Error 2: Use of an unquoted string.</p>';$some_var = tryetrhrtdf;echo '<p>Error 3: file not available:</p>';$fp = fopen('this_file_does_not_exist.dat','r');echo '<p>Error 4: db not available:</p>';$results = mysql_query('SOME QUERY');echo '<p>Error 5: call to undefined function:</p>';$fp = non_existing_function('an_arg');echo 'Code never gets here!!!';

http://ned.highline.edu/~tostrander/215/error_handling/error4.php

Hiding ErrorsHiding errors is NOT a

solution to a problem. It is useful, however, to

hide any errors produced on a live server.

While developing and debugging code, displaying all errors is highly recommended!

Suppressing ErrorsThe special @ operator can be

used to suppress function errorsAny error produced by the

function is suppressed regardless of the error reporting setting.

$db = @mysql_connect($h,$u,$p);if (!$db) {

trigger_error(‘blah’, E_USER_ERROR);}

Suppressing ErrorsError suppression is NOT a

solution to a problem.It can be useful to locally define

your own error handling mechanisms.

If you suppress errors, you must check for them yourself elsewhere.

Custom Error HandlingYou can write your own function to

handle PHP errors however you want.

The handler function should receive four arguments

The handler function should return true to indicate it has handled the error

Register the function in your script as the error handler

Custom Error Handlingfunction err_handler(

$errcode, $errmsg, $file, $lineno) {

echo ‘An error has occurred!<br />’;

echo “file: $file<br />”;

echo “line: $lineno<br />”;

echo “Problem: $errmsg”;

return true;

}

The handler must have 4 inputs..

1. error code2. error message3. file where error occurred4. line at which error

occurred

Custom Error HandlingThe function then needs to be registered

as your custom error handler:set_error_handler(‘err_handler’);

You can ‘mask’ the custom error handler so it only receives certain types of errors◦Example: register a custom handler just for

user triggered errors

set_error_handler(‘err_handler’,

E_USER_NOTICE | E_USER_WARNING |

E_USER_ERROR);

Custom Error HandlerA custom error handler is never

passed E_PARSE, E_CORE_ERROR or E_COMPILE_ERROR errors

These are considered too dangerous

function err_handler($errcode,$errmsg,$file,$lineno) {echo 'An error has occurred!<br />';

echo "file: $file<br />"; echo "line: $lineno<br />"; echo "Problem: $errmsg<br />";

return true;}

set_error_handler('err_handler'); // register handler

echo '<p>Error 1: Use of an undefined variable.</p>';echo $undefined_var;echo '<p>Error 2: Use of an unquoted string.</p>';$some_var = tryetrhrtdf;echo '<p>Error 3: file not available:</p>';$fp = fopen('this_file_does_not_exist.dat','r');echo '<p>Error 4: db not available:</p>';$results = mysql_query('SOME QUERY');echo '<p>Error 5: call to undefined function:</p>';$fp = non_existing_function('an_arg');echo 'Code never gets here!!!';

http://ned.highline.edu/~tostrander/215/error_handling/error5.php

PHP Programming 33

Other TipsTrace errorsFollow coding standardsWrite stub functions “Comment out” problematic linesAnalyze logic carefully

PHP Programming 34

Trace ErrorsTracing is the examination of

individual statements in an executing program

echo one of the most useful ways to trace PHP code

Use echo to display the contents of a variable, an array, or the value returned from a function

PHP Programming 35

function calculatePay() {

$PayRate = 15; $NumHours = 40;

$GrossPay = $PayRate * $NumHours;

echo “Gross Pay: $GrossPay<br />”;

$FederalTaxes = $GrossPay * .06794;

$StateTaxes = $GrossPay * .0476;

$SocialSecurity = $GrossPay * .062;

$Medicare = $GrossPay * .0145;

$NetPay = $GrossPay - $FederalTaxes;

echo “Net Pay 1: $NetPay<br />”;

$NetPay *= $StateTaxes;

echo “Net Pay 2: $NetPay<br />”;

$NetPay *= $SocialSecurity;

echo “Net Pay 3: $NetPay<br />”;

$NetPay *= $Medicare;

echo “Net Pay 4: $NetPay<br />”;

return number_format($NetPay, 2);

}

Stub Functions

function calcTax($amount)

{

return 1;

}

function isValid($email)

{

return true;

}Stubs return a hard-coded value

"Stubs" are empty functions that serve as placeholders for a program’s actual functions

PHP Programming 37

Use Comments to Debug$Amount = 100000;

$Percentage = .08;

printf(“The interest rate or a loan in the amount of

$%.2f is %s%%.<br />”, $Amount, $Percentage * 100);

$YearlyInterest = $Amount * $Percentage;

// printf(“The amount of interest for one year is

// $%.2f.<br />”, $YearlyInterest);

// $MonthlyInterest = $YearlyInterest / 12;

// printf(“The amount of interest for one month is

// $%.2f.<br />”, $MonthlyInterest);

// $DailyInterest = $YearlyInterest / 365;

// printf(“The amount of interest for one day is $

%.2f.

// <br />”, $DailyInterest);

The cause of an error in a statement is often the result of an error in a preceding statement

PHP Programming 38

Analyze LogicLogic errors are the hardest to

debugYou must analyze each statement

carefullyif (!isset($_GET['firstName'])) echo “You must enter your first name!”; exit();echo “Welcome to my Web site, ” . $_GET['firstName'];

PHP Programming 39

Analyze Logic

for ($count = 1; $count < 6; $count++);echo “$count<br />”;

$n = 1;while($n < 10){

echo $n;}

Isolate problematic

code

Learn Morehttp://www.w3schools.com/php/php_er

ror.asp