Functions with PHP

Introduction

Functions are the most important part of any programming language, PHP included. You can say in a few words that functions are pieces of code that accept values and produce results.

Function is a block of code that is not immediately executed, but can be called by your scripts when needed. Functions can either be built-in or defined by the user. The real power of PHP comes from its functions. In PHP, there are more than 700 built-in functions.

Functions help you create organized and reusable code. They allow you to abstract out details so your code becomes more flexible and more readable. Without functions, it is impossible to write easily maintainable programs because you're constantly updating identical blocks of code in multiple places and in multiple files.

Declare function

To declare a function, use the “function” keyword, followed by the name of the function and any parameters in parentheses. To invoke a function, simply use the function name, specifying argument values for any parameters to the function. If the function returns a value, you can assign the result of the function to a variable.

Syntax:  function <name of the function>([mixed $....]) { }

Example:

<?php

// Function Declaration

function myFunction() //no semicolon here!

{

print “<h1>This is my first function</h1>”;

}

// Call Function

myFunction();

?>

Note:

Ø  Function names are case-insensitive, though it is usually good form to call functions as they appear in their declaration.

Ø  All functions in PHP have the global scope - they can be called outside a function even if they were defined inside and vice versa.

Ø  Your function name can start with a letter or underscore "_", but not a number!

Variable Scope

Up to now you can used declared any variable anywhere in a page. With functions this is no longer always true. Functions keep their sets of variables that are distinct from those of the page and of page and of other functions.

The Variables defined in a function, including its parameters, are not accessible outside of the function, and by default, variables defined outside a function are not accessible inside the function.

<?php

  $text='Hey I’m in a pag';

?>

 <P>this is a web page<P>

<?php

 echo $text;

?>

<BR>

<?php

$a=3;

function Add()

{

$a+=2;

echo $a.'<BR>';

}

Add();

echo $a;

?>

Global Variable

If you want a variable in the global scope be accessible with in a function, you can use the global keyword.

<?php

$a=3;

function Add()

{

global $a;

$a+=2;

echo $a;

}

Add();

echo $a;

?>

You must include the global keyword in a  function before any uses of the global variable or variables you want to access. Because they are declared before the body of the function, Function parameters can never be globale.

Static Variables

Static variable is shared between all calls to the functions and it is initialized only the first time the function is, called.

Use static keyword to declare a static variable.

<?php

Function counter(){

static $count=0;

return $count++;

}

for($i=0;$i<=5;$i++)

{

echo counter ();

}

?>

Function arguments

Information may be passed to functions via the argument list, which is a comma-delimited list of expressions.

PHP supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists are also supported

Inside the function, it doesn't matter whether the values are passed in as strings, numbers, arrays, or another kind of variable. You can treat them all the same and refer to them using the names from the prototype.

You don't need to (and, in fact, can't) describe the type of variable being passed in PHP keeps track of this for you.

Making arguments be passed by value

All values being passed into and out of a function are passed by value, not by reference. This means PHP makes a copy of the value and provides you with that copy to access and manipulate. Therefore, any changes you make to your copy don't alter the original value.

<?php

// Function Declaration

function printName($name) //no semicolon here!

{

echo "Name is ".$name;

}

// Call Function with pass parameters by  values

 printName(“saman”);

?>

Making arguments be passed by reference

By default, function arguments are passed by value (so that if the value of the argument within the function is changed, it does not get changed outside of the function). To allow a function to modify its arguments, they must be passed by reference.

To have an argument to a function always passed by reference, prepend an ampersand (&) to the argument name in the function definition:

<?php

function add_some_extra(&$string)

{

    $string .= 'and something extra.';

}

$str = 'This is a string, ';

add_some_extra($str);

echo $str;    // outputs 'This is a string, and something extra.'

?>

Default argument values

<?php

//Function Declaration

//Parameters with Default values

function Mult($num1, $num2 = 1)

{

echo "$num*$num2";

}

//Function call with default argument

Mult(5);

//Function call with values

Mult(5,12);

?>

Exercise:

Define a function that takes 2 parameters (String and character representing text effect) and prints in that string with the text effect.

Default text effect should be “Bold”. If user wants, he can pass character for “Italics” and “Underline” too.

Creating Functions That Take a Variable Number of Arguments

Method 1: Using Arrays

function total($numbers) {

// initialize to avoid warnings

$sum = 0;

// the number of elements in the array

$size = count($numbers);

// iterate through the array and add up the numbers

for ($i = 0; $i < $size; $i++) {

$sum += $numbers[$i];

}

return $sum;

}

$tot = total(array(96, 93, 97));

Method 2: Using functions

function total() {

// initialize to avoid warnings

$sum = 0;

// the number of arguments passed to the function

$size = func_num_args();

// iterate through the arguments and add up the numbers

for ($i = 0; $i < $size; $i++) {

$sum += func_get_arg($i);

}

return $sum;

}

$tot = total(96, 93, 97);

This example uses a set of functions that return data based on the arguments passed to the function they are called from. First, func_num_args( ) returns an integer with the number of arguments passed into its invoking function.

From there, you can then call func_get_arg( ) to find the specific argument value for each position.

Exercise 5

Extend above to methods to get return the average of set of numbers.

Returning More Than One Value

Return an array and use list( ) to separate elements.

function time_parts($time) {

return explode(':', $time);

}

list($hour, $minute, $second) = time_parts('12:34:56');

You pass in a time string as you might see on a digital clock and call explode( ) to break it apart as array elements. When time_parts( ) returns, use list( ) to take each element and store it in a scalar variable.

If you want, You can also use global variables.

function time_parts($time) {

global $hour, $minute, $second;

list($hour, $minute, $second) = explode(':', $time);

}

time_parts('12:34:56');

Skipping Selected Return Values

A function returns multiple values, but you only care about some of them.

Omit variables inside of list( )

function time_parts($time)

{

return explode(':', $time);

}

list(, $minute,) = time_parts('12:34:56');

Exercise

Extend the above example so that time part HOURS get returned and to display some greeting message based on the time. Eg: if hrs<12, Good Morning.

Calling Variable Functions

function eat_fruit($fruit)

{

print "chewing $fruit.";

}

$function = 'eat_fruit';

$fruit = 'kiwi';

$function($fruit); // calls eat_fruit( )

Exercise

Define three different functions that take two numbers as input parameters and print addition, subtraction and multiplication respectively.

Outside to functions, do following steps.

Declare two variables and assign them with 10 &15.

Generate random number.

HINT: To generate a random number between two end points, pass mt_rand( ) two arguments:

$random_number = mt_rand(1, 100);

If random number is <20, you should get printed the addition of above two numbers (10 &15)

If 20<=random number <50, you should get printed the substraction of above two numbers (10 &15)

If random number is >50, you should get printed the multiplication of above two numbers (10 &15)

RULE: YOU CAN USE ONLY ONE FUNCTION CALL INSIDE YOUR PROGRAM.

Creating Dynamic Functions

Use create_function( )

$add = create_function('$i,$j', 'return $i+$j;');

$add(1, 1); // returns 2

The first parameter to create_function( ) is a string that contains the arguments for the function, and the second is the function body.

Internal (built-in) functions

Date manipulation function

Get Current date (date() function)

<?php

$today = date("F j, Y");

echo "$today";

$today = date("d / M/ Y");

echo "$today";

?>

Write a program to get the date in 12th December 2008 like?

<?php
// Assuming today is: March 10th, 2001, 5:16:18 pm

$today = date("F j, Y, g:i a");                 // March 10, 2001, 5:16 pm
$today = date("m.d.y");                         // 03.10.01
$today = date("j, n, Y");                       // 10, 3, 2001
$today = date("Ymd");                           // 20010310
$today = date('h-i-s, j-m-y, it is w Day z ');  // 05-16-17, 10-03-01, 1631 1618 6 Fripm01
$today = date('\i\t \i\s \t\h\e jS \d\a\y.');   // It is the 10th day.
$today = date("D M j G:i:s T Y");               // Sat Mar 10 15:16:08 MST 2001
$today = date('H:m:s \m \i\s\ \m\o\n\t\h');     // 17:03:17 m is month
$today = date("H:i:s");                         // 17:16:17
?>

Find dates in the future or the past

<?php

$tomorrow  = mktime(0, 0, 0, date("m")  , date("d")+1, date("Y"));

$lastmonth = mktime(0, 0, 0, date("m")-1, date("d"),   date("Y"));

$nextyear  = mktime(0, 0, 0, date("m"),   date("d"),   date("Y")+1);

echo date('d/M/y',$tomorrow)."<br>";

echo date('d/M/y',$lastmonth)."<br>";

echo date('d/M/y',$nextyear)."<br>";

?>

Note: This can be more reliable than simply adding or subtracting the number of seconds in a day or month to a timestamp because of daylight saving time.