Web Programming: Handling Files and Exceptions

Web Programming
Handling files and exceptions

Files and their uses
 Storing text (including
error logs) / images /
HTML templates
XML
 Universal way to Application 1 Application 2
exchange structured JSON
data, e.g. JSON, XML
 Exchanging tabular
CSV Spreadsheet
data – CSV Database
File function examples
file_get_contents, Reading / writing files
file_put_contents, file,
fopen, fgets, fgetcsv,
feof, fwrite, fputcsv,
fclose
file_exists, filesize, Checking / asking for

is_readable, information
disk_free_space
Writing and reading a file ex
Application 1 writes
a file of employee employees
Application 2 reads
details and makes
the file
it available to
application 2
Writing data to a file
 We can use: fwrite
 You need to open the file with a mode that permits
writing
Open file in append writing mode
 Example
$fileHandle = fopen("employees.txt", "ab");
$employee = "Another person";
$dept = "MCE";
Add new
fwrite($fileHandle, "$employee|$dept".PHP_EOL);
line
fclose($fileHandle);
 We could of course write to a file using data dynamically

extracted from an external source like a database if required
Logging errors
 How could we make a function that stores
errors in a log file?
Function, error_log_file.log
e.g., log_error($e) Date/time of error|error details
Date/time of error|error details
Write error date/time Date/time of error|error details
and error details to Date/time of error|error details
a file …
fopen file modes
 The first argument to fopen is the name of
the file you want to access
 The second is the file mode. These include
Mode Action(s)
rb Reading from the beginning of a file
rb+ Reading and writing from the beginning of a file
wb Writing from the beginning of a file
wb+ Reading and writing from the beginning of a file
ab Writing from the end of a file
ab+ Reading and writing from the end of a file
Note that the “b” is for binary

compatibility
Reading a file line by line
 We can use: fopen, fgets, feof and fclose. Example
 Open file and return a connection handle
$fileHandle = fopen("employees.txt", "rb");
 While not the end of the file, read a line

while (!feof($fileHandle)) {
$line = fgets($fileHandle);
if($line) {
$line = trim($line);
$part = explode('|', $line);
echo "<li>$part[0], $part[1]</li>\n";
// or e.g., write data to a database record
}
}
fclose($fileHandle);
Dan Hogdson|CIS\n
Garry Elvin|CIS\n
employees.txt
Admin bod|HR\n
CSV files
 CSV files allow sharing of tabular data among programs
 We can use:
 fgetcsv – reads a line from a CSV file and returns it as an array
containing each field in the line
 fputcsv – takes an array of values and writes them as properly
formatted CSV to a file
 We could, for example:
 Retrieve data from a database and write it as CSV-formatted
data to a file using fputcsv
 Read data from a CSV-formatted file using fgetcsv and write it to
a database
 The workshop directed learning tasks will cover CSV

Errors and exception handling
 Things go wrong!
 Exceptions are used to anticipate and handle
problems gracefully using try{} catch{}
blocks
 They are thrown when an irreparable
situation is encountered and are then caught
 They can be used both offensively and
defensively
Throw exceptions
 Throw using the Exception object
 Exception sub-classes include, e.g.,
 Exception, ErrorException, InvalidArgumentException
 Example of bad practice
$num1 = 2;
$num2 = "Garry";
if (!is_numeric($num1) || !is_numeric($num2)) {
throw new Exception("num1 and num2 are not both
numeric");
}
$result = $num1 / $num2;
Catch exceptions
 Thrown exceptions need to be caught and
handled gracefully
 As uncaught exceptions terminate the
application with a fatal error and can expose
sensitive debugging details
 So, surround code that might lead to an
exception with a try / catch block
Example with catching
try {
$num1 = 2;
Snum2 = "Garry"; Not an number! So we get an exception
throw new Exception("num1 and num2 are not both numeric");
}
$result = $num1 / $num2;
} catch (Exception $e){
// In a system in test mode we can echo exceptions
echo "A Problem occurred: ".$e->getMessage();
/* In a production system we could write them to a log file
instead, e.g.,
log_error($e); */
}
getMessage() is a method of
Exception that accesses system
generated exception details
An example involving PDO
try {
require_once("functions.php");
$dbConn = getConnection(); An exception could
be thrown by the
/* Code to query a database etc. function or by the
could go here */ PDO code to query
etc.
/* Take some appropriate action. */
}
Wherever an exception is thrown in the try block, the catch

block will handle it
Additional Exception sub-
classes
 The SPL provides additional sub-classes for
certain situations
LogicException RuntimeException
BadFunctionCall Exception OutOfBoundsException
•BadMethodCallException OverflowException
DomainException RangeException
InvalidArgumentException UnderflowException
LengthException UnexpectedValueException
OutOfRangeException
Sub-class example
function divide ($num1, $num2) {
throw new InvalidArgumentException("Function
only accepts numbers");
}
return $num1 / $num2;
}
 Example adapted from php.net
Sub-class example in a function (note that you can throw

using Exception in a function as well if you need to)
Catching multiple exceptions
 You can intercept more than one type of
exception with multiple catch blocks. Example
try {
echo divide("A", 2);
} catch (InvalidArgumentException $e){
// Take some appropriate action
/* Handle any other exceptions by taking some
appropriate action */
}
Note that only one catch is triggered per exception

More than one sub-class
function divide ($num1, $num2) {
throw new InvalidArgumentException("Function
only accepts numbers");
}
if ($num2 == 0) {
throw new RangeException("Divisor cannot be
zero");
}
return $num1 / $num2;
}
Example adapted from php.net

Catching multiple exceptions
 Adding another catch block
try {
echo divide(4, 0);
} catch (RangeException $e){
// Handle any other exceptions
}
finally block
 You can add a finally block that will always run if there where any
exceptions or not. Example
try {
echo divide(4, 0);
} catch (RangeException $e){
// Handle any other exceptions
} finally {
// Put code here that will always run
}
Handling uncaught exceptions
 We should always include try/catch around any code that may throw an
exception. However, its hard to do perfectly
 So, set a global exception handler as a safety net
 To do that you can create a custom exception handler function, e.g.,
function exceptionHandler ($e) {

/* Code to handle uncaught exceptions by taking some
appropriate action */
}
 and register it, e.g.,

set_exception_handler('exceptionHandler');
ErrorException
 PHP errors can occur for a number of reasons, including generation by
older functions (like fopen) that still trigger errors rather than exceptions,
etc.
 We can bring errors within exception handling by using ErrorException
objects. Example
/* Check if the file exists before trying to open it. If

it doesn’t, throw an ErrorException */
if(!file_exists("employees.txt")) {
throw new ErrorException("Couldn't open file");
}
$fileHandle = fopen("employees.txt", "rb");
/* The code to use the open file connection would go here */

ErrorException – global handler
 We should set a global error handler function as a safety net for PHP
errors to help ensure that all errors are brought within exception
handling by converting them into ErrorException objects
 To do that you can create a custom error handler function, e.g.,
function errorHandler ($errno, $errstr, $errfile, $errline) {

if(!(error_reporting() & $errno)) { // check error isn’t excluded by server settings
return;
}
throw new ErrorException($errstr, $errno, 0, $errfile, $errline);
}
set_error_handler('errorHandler');
Development and production
 Development
 During development it is essential to display
detailed exception messages to the browser
 Production (when a system is live)
 Display only user friendly messages like
“Something went wrong. Please try again”
 Log detailed exception information to a file for
later inspection
Development and production
Define a constant that indicates development status in a script, e.g.,
setEnv.php
define("DEVELOPMENT", true);
Script that needs to know the development status

require_once("setEnv.php");
…
if (defined("DEVELOPMENT") && DEVELOPMENT===true) {
/* Code to display detailed exception message,
including, e.g., $e->getMessage(), $e->getFile(),
and $e->getLine() */
} else {
// Code to display a user friendly message
// Code to log detailed exception info to a file
}

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Web Programming

Handling files and exceptions

file_exists, filesize, Checking / asking for

 We could of course write to a file using data dynamically

Note that the “b” is for binary

 While not the end of the file, read a line

 The workshop directed learning tasks will cover CSV

Wherever an exception is thrown in the try block, the catch

 Example adapted from php.net

Sub-class example in a function (note that you can throw

Note that only one catch is triggered per exception

Example adapted from php.net

function exceptionHandler ($e) {

 and register it, e.g.,

/* Check if the file exists before trying to open it. If

/* The code to use the open file connection would go here */

function errorHandler ($errno, $errstr, $errfile, $errline) {

Script that needs to know the development status

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