1. PHP: Further Skills By Trevor Adams

2. Topics covered Brief re-cap of core PHP characteristics Arrays Loops Code organisation Error handling Online resources

3. Brief Overview of Core PHP Variables $ sign, weakly typed Constants Conditional branching If statement Switch statement Logical comparison Interacting with browser $_POST $_GET HTML elements

4. 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 $_POST[“txtMsg”] $soldiers[0]

5. 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(); $myarray[0] = “Trevor Adams”; echo $myarray[0]; Prints “Trevor Adams”; Or: $myarray = (0 => “Trevor Adams”); echo $myarray[0]; Prints “Trevor Adams”;

6. Arrays - Characteristics Arrays can store any type of variable, including other arrays. E.g. $myarray = array(); $myarray[0] = array(); $myarray[0][0] = “Trevor Adams”; echo $myarray[0][0]; Prints “Trevor Adams” There is no size limit Is limited by server capacity Be nice to your server

7. 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

8. 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

9. 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

10. 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!

11. 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)…

12. Loops - for $mulitple = 5; for($counter = 1; $counter<13; $counter++){ $answer = $multiple * $counter; echo “$counter * $multiple = “. $answer.” ”; } 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

13. 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

14. Loops - foreach The foreach loop is available in two formats Assume the following array: $campus[“sot”] = “Stoke-on-Trent”; $campus[“sta”] = “Stafford”; $campus[“lic”] = “Lichfield”; A foreach loop can be used to iterate the entire array

15. Loops - foreach in use First method allows us to get the values. E.g. foreach($campus as $campusname) { echo $campusname. “ ”; } Would output: Stoke-on-Trent Stafford Lichfield

16. Loops - foreach in use The second method allows access to the key: foreach($campus as $key => $name) { echo “$key - $name ”; } Would produce output: sot - Stoke-on-Trent sta - Stafford lic - Lichfield This loop is excellent at traversing arrays of elements of unknown quantity

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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 “ $title ”; echo “ ”; } Function print_footer() { echo “ ”; }

23. Functions - defining and using <?php print_header(“Sample page”); echo “ Hello World! ; print_footer(); ?> Would produce: Sample page Hello World!

24. 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.

25. 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: <?php include(“utils.php”); print_header(“My Homepage”); echo “ Hello World! ”; print_footer(); ?> Require can be used instead of include Require causes a page error however, if the file to include cannot be found

26. 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

27. 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

28. 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

29. PHP Errors - Notices A notice is not actually an error at all It merely serves as information to the programmer <?php echo " $message "; ?> Produces:

30. PHP Errors - Notices You know that @ 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 <?php If(isset($message)) { echo “ $message ”; } ?> Testing the variable for a value removes the warning message from the script

31. PHP Errors - Syntax Errors Syntax errors are easy to spot. Usually caused by inputting code incorrectly eco “ $message ”; Use the line number to find the error and correct it

32. 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

33. 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

34. 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;

35. 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()

36. 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

37. Online resources PHP web site - http://www.php.net/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.