PHP: Further Skills

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PHP Further Skills

• By Trevor Adams

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Topics covered

• Brief re-cap of core PHP characteristics
• Arrays
• Loops
• Code organisation
• Error handling
• Online resources

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Brief Overview of Core PHP

• Variables
• sign, weakly typed
• Constants
• Conditional branching
• If statement
• Switch statement
• Logical comparison
• Interacting with browser
• _POST
• _GET
• HTML ltformgt elements

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Arrays - Simple collections

• It is common to group related data
• Ordered set of related elements
• Arrays in PHP are very simple
• Weakly typed just like variables
• Accessed using brackets
• You encountered an array when posting a form
• _POST is an array provided by PHP to present
  form data
• Access array elements using a key
• Either numerical or text based key
• _POSTtxtMsg
• soldiers0

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Arrays - declare and use

• Defining an array is similar to a variable
• Same rules apply to array variable names
• Can be declared empty or initialised
• For example
• myarray array()
• myarray0 Trevor Adams
• echo myarray0
• Prints Trevor Adams
• Or
• myarray (0 gt Trevor Adams)
• echo myarray0
• Prints Trevor Adams

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Arrays - Characteristics

• Arrays can store any type of variable, including
  other arrays. E.g.
• myarray array()
• myarray0 array()
• myarray00 Trevor Adams
• echo myarray00
• Prints Trevor Adams
• There is no size limit
• Is limited by server capacity
• Be nice to your server

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Arrays - working with data

• Arrays are collections of related data
• Sometimes necessary to process this data
• Need a way of iterating through an array
• Arrays need repetitive operations
• This means Loops!
• With out further a do
• we shall come back to arrays soon

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Loops - iteration made easy

• Many tasks involve iteration
• The repetition of a process
• Computer programs do the job well
• PHP provides us with choices
• While
• Do
• For
• foreach
• Each of them are useful in different ways

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Loops - While

• While loops are the most simple loop type
• Similar in syntax to an IF statement
• Code runs while the test expression is true
• E.g.
• While(valid true)
• //execute commands
• //test the test expression
• BE SURE to make sure your code can exit the loop
• At some point, valid must not equal true to break
  the loop
• Avoid infinite loops, they ruin everyone's day

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Loops - Do

• Do loop is extremely similar to a while loop
• Major difference, text expression is at the end
• This loop will always execute at least once
• E.g.
• Do
• //execute these statements at least once
• //another statement
• while(something true)
• Again, make sure your loop can end!

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Loops - for

• For loops are best when the number of iterations
  are known
• Using pseudo code
• for(set loop counter test loop counter adjust
  loop counter)
• // execute statements within braces
• Trivial example would be to print out a
  multiplication table (up to 12)

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Loops - for

• mulitple 5
• for(counter 1 counterlt13 counter)
• answer multiple counter
• echo counter multiple . answer .ltbr
  /gt
• This code would result in output
• 1 5 5
• 2 5 10
• 3 5 15 .. And so on
• Ensure you do not erroneously adjust counter
  during the loop

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Loops - foreach

• We get back to arrays!
• The foreach loop is an extension of the for loop
• Except you do not need to know the number of
  elements
• It can move through an array for you
• Extremely useful loop for traversing arrays

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Loops - foreach

• The foreach loop is available in two formats
• Assume the following array
• campussot Stoke-on-Trent
• campussta Stafford
• campuslic Lichfield
• A foreach loop can be used to iterate the entire
  array

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Loops - foreach in use

• First method allows us to get the values. E.g.
• foreach(campus as campusname)
• echo campusname . ltbr /gt
• Would output
• Stoke-on-Trent
• Stafford
• Lichfield

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Loops - foreach in use

• The second method allows access to the key
• foreach(campus as key gt name)
• echo key - nameltbr /gt
• Would produce output
• sot - Stoke-on-Trent
• sta - Stafford
• lic - Lichfield
• This loop is excellent at traversing arrays of
  elements of unknown quantity

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Arrays and Loops - overview

• Arrays - collections of related data
• Act like variables
• Access sub-elements using square brackets
• Can index elements with numerical or textual keys
• Loops - useful for repetitive processing
• while and do test a Boolean expression
  (something true)
• for loops require a counter
• foreach loops traverse arrays

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Code Organisation

• All about being efficient and productive
• Skills you should possess as a programmer
• Creating functions
• Calling functions
• Scope
• Modular code
• Placing code in separate files
• Make commonly used code available

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Functions - defining and using

• We create functions for a number of reasons
• Avoid repetition (E.g. frequently used
  calculations)
• Easier to test small code fragments
• Functions have a name, and optionally may take
  arguments
• Functions may merely execute commands
• They might return a value
• Completely up to the programmer

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Functions - defining and using

• You can declare a function anywhere inside PHP
  code blocks
• A function is declared using the keyword
  function. E.g.
• function sum_numbers (a, b)
• return a b
• This function could then be used in PHP code.
  E.g.
• num1 4
• num2 6
• myAnswer sum_numbers(num1, num2)
• echo myAnswer // would show 10

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Functions - defining and using

• function sum_numbers (a, b)
• return a b
• a and b in this function are arguments
• They exist inside the function braces only
• It is said they have local scope
• Values you pass to a function are not affected
  outside of it

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Functions - defining and using

• A function does not have to return a value
• Possible to use output text
• E.g. creating a HTML header and footer function
• function print_header(title)
• echo lthtmlgtltheadgtlttitlegttitlelt/titlegtlt/headgt
• echo ltbodygt
• Function print_footer()
• echo lt/bodygtlt/htmlgt

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Functions - defining and using

• lt?php
• print_header(Sample page)
• echo ltpgtHello World!lt/pgt
• print_footer()
• ?gt
• Would produce
• lthtmlgtltheadgtlttitlegtSample pagelt/titlegtlt/headgtltbody
  gt
• ltpgtHello World!lt/pgt
• lt/bodygtlt/htmlgt

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Code - Include / Require

• PHP files do not have to be web pages
• It is possible to have a PHP made completely of
  functions
• If this page was to be called in a browser, it
  would be completely blank
• Including other files within a PHP script is a
  common form of modularisation
• Common with many server side scripting languages
• Allows the programmer to place code required in
  many places into one location, available to all
  scripts.

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Code - Include / Require

• PHP uses the include function and the require
  function to incorporate other files
• Place our print_header and print_footer in a file
  called utils.php
• We create index.php - we can then use
• lt?php
• include(utils.php)
• print_header(My Homepage)
• echo lth1gtHello World!lt/h1gt
• print_footer() ?gt
• Require can be used instead of include
• Require causes a page error however, if the file
  to include cannot be found

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Code Organisation - Overview

• Functions allow you to separate your frequently
  used code
• Promotes good practice
• Code re-use
• Less typing!
• Include files allow a programmer to create useful
  libraries of code
• Promotes further good practice
• Allows you to create global code

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Error Handling with PHP

• We should now have enough basic programming
  knowledge to consider errors
• Error handling is perhaps one of the main weak
  points of PHP
• We shall be covering
• Why we care
• Notices
• Syntax errors
• Program errors
• Logical errors
• How to handle and avoid errors

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Why do we care about errors?

• Security - paramount to any web application
• Data comes from non-trusted sources (users!)
• Web messages
• Standard programs have the ability to pipe error
  messages in many ways (log, dialog etc.)
• PHP only has the option of outputting messages to
  the browser
• It does so, you might have already seen messages
  in your own code
• Nothing says amateur like rogue error messages in
  a production application

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PHP Errors - Notices

• A notice is not actually an error at all
• It merely serves as information to the programmer
• lt?php
• echo "lth1gtmessagelt/h1gt"
• ?gt
• Produces

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PHP Errors - Notices

• You know that _at_ suppresses the message
• A better technique is to test the variable first
• Use the isset() function or the empty() function
• isset(varname) returns true if the variable has
  been given a value
• empty(varname) returns true if it is null
• lt?php
• If(isset(message))
• echo lth1gtmessagelt/h1gt
• ?gt
• Testing the variable for a value removes the
  warning message from the script

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PHP Errors - Syntax Errors

• Syntax errors are easy to spot.
• Usually caused by inputting code incorrectly
• eco lth1gtmessagelt/h1gt
• Use the line number to find the error and correct
  it

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PHP Errors - Program Errors

• These errors occur when a error happens that is
  generally unforeseen
• Trying to include a file which does not exist
• Trying to connect to a database that is offline
• Results of functions should generally be checked
  and handled well
• Solutions to these issues are often complex
• We shall return to these errors later in the
  module

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PHP Errors - Logical Errors

• Hardest type to spot!
• Typical example is a program script attempting to
  divide by zero
• This could happen in a for loop for example
• Read the errors on screen
• Line numbers are useful
• Messages are meaningful
• Take the time to think about the message

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PHP Errors Top causes

• Typing mistake - check your spelling first
• Construct improperly closed
• E.g. missing a brace
• Use comments to help
• Missing a semi-colon after a statement
• Getting the name of a function wrong
• E.g. eco Hello
• Not closing a string
• E.g. echo Hello

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PHP Error Handling - overview

• Topics covered
• Why we bother with error handling
• Types of errors
• Notices
• Syntax
• Program errors
• Logical errors
• How to handle errors
• Checking with isset() and empty()

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Topics covered this lecture

• Arrays
• A type of variable, containing related items
• Accessed via keys, numerical or textual
• Values accessed using square brackets
• Code Organisation
• Why we use functions
• The ability to include external files
• PHP Error Handling
• Why we bother with errors
• Different types of errors
• Top causes of errors

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Online resources

• PHP web site - http//www.php.net/
• Contains a searchable database of PHP functions
• Try searching for sort in the function list
• You will be presented with an impressive list of
  PHP functions involving sorting arrays
• Learn to use the PHP manual - it is an invaluable
  resource for programming PHP.