Class 3 - PHP Functions

Outline
• Functions
• Function declaration
• Function Arguments
• Variable number of arguments
• Returning Values
• Variable Scope
• Static variables
• Recursion
• Tricks
• Useful Built-in Functions
• Assignment

Functions
• Is a sub program.
• A block of code that is defined under a certain name
( function name ).
• Can take arguments.
• Can return a value.
• Function name has the same naming rules as variables
and constants.
• Function names are not case sensitive.

Function declaration
function doSomething($arg_1, $arg_2, /* ..., */ $arg_n)
{
echo “something.n";
return $val;
}
Name Arguments
Body
Return

Function Arguments
• Passing by value (default – except for objects) :
function multiply($x, $y)
{
echo $x * $y;
}

Function Arguments
• Passing by reference :
$x = 10;
$y = 5;
$result = 0;
multiply( $x, $y , $result );
echo $result; // 50
function multiply($x, $y, &$result)
{
$result = $x * $y;
}

Function Arguments
• Default argument values:
function doIt( $name = “PHP” )
{
echo “Hello ” . $name;
}
Can be called :
doIt(); // Hello PHP
doIt( “Guys” ); // Hello Guys
• It is preferable that the arguments with default values
should be the last arguments passed to the function.

Function Arguments
• PHP emits a warning if the number of the arguments
passed is not the same the number of arguments in the
function declaration.
• PHP does not throw any warning in case the number is
bigger than the arguments in the function declaration.
function doIt( $name )
{
echo “Hello ” . $name;
}
doIt(); // a warning is emitted
doIt( “Elephant”, “Cow” ); // this is OK

Variable number of arguments
• PHP allows you to create a function with unlimited
number of arguments.
function displayAll( )
{
$numargs = func_num_args();
$arg_list = func_get_args();
for ($i = 0; $i < $numargs; $i++) {
echo $arg_list[$i] . “ ”;
}
}
displayAll( “This” , “is”, “a”, “good”, “thing.” );

Returning Values
• Functions may return values :
function doIt( $name )
{
return “Hello ” . $name;
}
echo doIt(“PHP”); // Hello PHP

Variable Scope
$a = 1; /* global scope */
function test()
{
echo $a; /* reference to local scope variable */
}
test(); // echoes an empty string

Variable Scope
The GLOBAL keyword:
$a = 1; /* global scope */
function test()
{
global $a; // can also use the $GLOBALS array
echo $a; /* reference to local scope variable */
}
test(); // 1

Static variables
• Static variables are initialized only one time.
function test()
{
static $a = 0;
echo $a;
$a++;
}
test(); //0
test(); // 1
test(); // 2

Recursion
• Recursion occurs when something contains, or uses, a
similar version of itself.
• In programming, it happens when a function is having
a call to itself.
• The factorial problem :
• 5! = 5 x 4 x 3 x 2 x 1 = 120

Recursion
function factorial($number) {
if ($number == 0) return 1;
return $number * factorial($number - 1);
}
echo factorial(6); // 720

Tricks
A function inside an IF statement :
if ($condition) {
function doSomething()
{
echo “Will only be defined if the IF condition is true";
}
// some code
doSomething();
}

Tricks
A function inside another function:
function doSomething1()
{
function doSomething2()
{
echo "I can only be called from inside doSomething1
n";
}
// some code
doSomething2();
}

Tricks
Variable functions:
function display( $x )
{
echo $x;
}
$name = “display”;
$name( “PHP” ); //PHP

Tricks
Anonymous functions:
$greet = function($name)
{
echo “Hello “ . $name;
};
$greet(“PHP”); // Hello PHP

Useful Built-in Functions
• boolean function_exists( $name )
• Checks whether there is a function with that
$name.
• void get_defined_functions()
• Gets a list of all names of defined functions in the
script.

Assignment
- Write a PHP function that calculates the average of passed
parameters (The number of parameters is dynamic).

Class 3 - PHP Functions

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