1. ITM 352 Functions

2. Functions
A function is a named block of code (i.e. within {}'s) that performs a specific set of statements
It acts on a set of values given to it (parameters)
It may return a single value (return-value)
Functions are very useful for
Repeated tasks in multiple locations
Sharing useful code
Saving execution time
Modularization: Dividing up the task

3. Functions We've already made use of several functions!
Can you think of some?
Some functions are built in to PHP, some are added from code libraries, some you will define

4. Calling a Function
All PHP functions are designated by <identifier>(<parameters>)
e.g. phpinfo(), rand(1,10)
Function names are not case sensitive
Full interface for a function:
<return type> <identifier> ( <parameters> )
identifier
parameters

5. Calling a Function Functions are used by calling them <?php …
$commission = $sales * $rate;
echo round($commission,2);
?>
the "call"

6. Passing Values into a Function: Parameters
Some Functions can be more flexible (therefore useful) if we pass them input values to use in their operations
Input values for functions are called passed values or parameters
Parameters must be specified inside the parentheses () of the function, and must be of the expected data type, in the expected order as defined by the function's interface
rand(int min, int max)
rand(1,10);
Parameter 2: max (int)
Parameter 1: min (int)

7. rand($minGuess,$maxGuess);
Passing Variables
Any legal expression can be used as a parameter (recall that an expression returns a value)
rand(44%3, 2*$max*$max);
It is very common to use a variable for a parameter (recall: a variable by itself is an expression – it returns the value of the variable)
rand($minGuess,$maxGuess);
substr($name,1,$i);

8. Return Values of Functions
Some Functions just perform an action (e.g. read in a value from the keyboard, switch to a web page) and do not return a value
Most functions perform an action and return a single value
Return types may be:
a primitive data type, such as int, float, boolean, etc.
a compound type such as array, object, etc.
void if no value is returned
Return values are how a function passes information back after it is called and after it performs its operations.

9. Return Values of Functions
You can use a function with a returned value any place where it is legal to use an expression,
$guess = rand(1,10);
*rand(1,10);
echo phpinfo();
$yummy = "piece of " . substr("piece",0,3);
if( is_int($guess) ) …
A common return value for a function is boolean
true is the function operations were successful with no problems
false if the function failed to perform its task
if( !writeFile() ) echo "file not written";

10. Function Documentation
Functions are documented not only with a description of the function, but also with a interface that shows the return value type, the name of the function, and the required and optional arguments:
type function_name (type arg1, type arg2 [,type optional_arg])
PHP functions often document functions this way:
printf
(PHP 3, PHP 4 )
printf -- Output a formatted string
Description
void printf ( string format [, mixed args]) Outputs a string by using format and the given arguments …
function name
function signature
function description

11. ITM 352 Creating Functions

12. Defining Functions
A very important aspect of PHP is the ability to create your own functions. You will find out how useful this is later.
You declare a function with the function keyword in front of a identifier and function parameter set and a code-block:
function <identifier> ( <parameters> ) {
// function operations (code statements) here
return <expression> }
The way you define a function defines the functions interface (or how it is expected to be used)

13. Function Syntax and Example
function funcname($var1, $var 2...) {
statement 1;
statement 2;
statement 3;
...
return $retval; }
function square($num) {
$answer = $num * $num;
return $answer; }
To call the function:
$quantity = 3;
$myvar = square ($quantity);
$myvar will contain the value 9
NOTE that the variable names do not need to be the same; the order of variables passed is what will matter inside the function!

14. Global vs. Local Variables
Variables defined inside a function (or passed in) are "local variables" and are only available within functions
after exiting the function the variable ceases to exist!!!
Variables defined outside of a function are generally not available inside a function. So variables should be passed into a function as arguments!!!
Global variables are accessible both in and out of functions
NOT recommended!!! DO NOT DO IT!!!!
Where the variable is active (alive) is known as its "scope"
use global and static to modify this
also "passing by reference" will affect scope

15. Scope of Variables
Inside a function is like an entirely new program!!!
Variables that you define within the function have "local" scope:
They don't exist before the function starts
They don't exist once the function has completed

16. What will this program print?
Scope of Variables – 2
<?php
function deposit($amount) {
$balance = $balance + $amount;
echo "New balance is $balance <BR>"; }
$balance = 600;
deposit(50);
echo "Balance is $balance";
?>
What will this program print?
Why?

17. What would happen if we used $a and $b instead?
Defining Functions
function swapTwoValues($a, $b) {
$tmp = $a; // save old value temporarily
$a = $b; // copy second parameter into first
$b = $tmp; // copy original first value into second }
$var1 = 1; $var2 = 2;
echo "\$var1 is $var1 and \$var2 is $var2<BR>";
swapTwoValues($var1, $var2);
echo "\$var1 is $var1 and \$var2 is $var2";
What would happen if we used $a and $b instead?

18. Function Naming Conventions
Good Programming Practice (we will look for this!)
Use verbs to name functions that do not return values or operate directly on variables
They usually perform an action e.g. sort(), print_table()
Use nouns to name functions that return a value
they create (return) a piece of data, a thing e.g. date()
Start function names with a lower case letter
phpinfo()
Use "_" to separate words
is_bool()
Use descriptive names
get_html_translation_table()

19. Pass-By-Value vs. Pass-By-Reference
When a function is called, the value of each argument is copied (assigned) and used locally within that function
Variables used as arguments cannot be changed by a function!!!!!!!!!
One way to change a value of a passed in variable is to make use of return value:
function doubler($value) {
return 2*$value; }
echo doubler($doubleUp);
$doubleUp = doubler($doubleUp);

20. Pass-By-Value vs. Pass-By-Reference
Sometimes it's more convenient to directly operate on a variable, so you pass in its reference (address):
function swapTwoValues(&$a, &$b) {
$tmp = $a; // save old value temporarily
$a = $b; // copy second parameter into first
$b = $tmp; // copy original first value into second }
$var1 = 1; $var2 = 2;
echo "\$var1 is $var1 and \$var2 is $var2<BR>";
swapTwoValues($var1, $var2);
echo "\$var1 is $var1 and \$var2 is $var2";