function for tutorial

tutorial-function.md

Functions

PHP already has a lot of built-in functions. Besides the built-in functions already provided by PHP it is also possible to define your own functions. Functions are used to group code into something useful, it organizes your code.

Generally speaking a function is a piece of code with an input, some defined actions based on the input and an output and then an optional return value:

INPUT --> PROCESSING --> OUTPUT

A function therefore is a little program inside a larger program. It sometimes is called subroutine as well because of that.

An example of function that calculates the differences in the age of two people is:

function getAgeDifference($age1, $age2)
{
    return abs($age1 - $age2);
}

$users = array(
    array(
        'name' => 'John Doe',
        'age'  => 24,
    ),
    array(
        'name' => 'Jane Doe',
        'age'  => 36,
    ),
);

echo 'The difference of age in years between ' . $users[0]['name'] . ' and ' . $users[1]['name'] . 'is: '
     . getAgeDifference($users[0]['age'], $users[1]['age']) . ' years.';

This example will output:

The difference of age in years between John Doe and Jane Doe is: 12 years.

What happens in this example is that we pass two parameters to the function (i.e. $age1 and $age2) and that the function calculates the differences between the two dates:

function getAgeDifference($age1, $age2)
{
    return abs($age1 - $age2);
}

Also note that in this example we have used the built-in function abs(). The abs() function return an absolute number. If we wouldn't have added that the result of our function would have been -12 instead of 12. Also note the return keyword in our function. The return keyword in a function ends the execution of the function and returns a value (in our case the number of years in between the two ages).

Now that we have seen a simple example of a function lets have a look at a bit more complex function. Lets create a function which creates an excerpt from a longer text:

function getExcerpt($text, $numberOfCharacters = 200)
{
    $excerpt = mb_substr($text, 0, $numberOfCharacters);

    if (mb_strlen($text) > $numberOfCharacters) {
        $excerpt.= '...';
    }

    return $excerpt;
}

$articles = array(
    '1' => array(
        'timestamp' => '2012-02-06 16:43:15',
        'title'     => 'Blog example',
        'text'      => 'Morbi viverra lectus sit amet diam feugiat convallis. Fusce ipsum tellus, porta convallis bibendum a, sagittis id arcu. Etiam at enim nisi. Sed luctus aliquet varius. Aenean pharetra tortor ac ipsum mollis quis suscipit lacus porta. Etiam ligula nibh, vestibulum at blandit vel, facilisis eu nulla. Curabitur eget urna lectus, sed accumsan ante. In vestibulum neque ut dui mollis vitae aliquam enim venenatis. Mauris vitae sapien ligula. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Suspendisse potenti. Morbi bibendum molestie elit, non vestibulum risus consequat sagittis. In hac habitasse platea dictumst. Curabitur vehicula tempor lacus lobortis venenatis.',
    ),
    '2' => array(
        'timestamp' => '2012-02-07 14:12:21',
        'title'     => 'Second blog post',
        'text'      => 'Maecenas quis lectus sed tortor consectetur sodales eu sed augue. Curabitur non justo suscipit arcu sollicitudin adipiscing mattis luctus elit. Praesent venenatis pretium justo, eget fringilla urna laoreet sit amet. Donec ullamcorper metus at lacus semper ut rutrum ante sagittis. Suspendisse potenti. Proin et quam justo. Nulla leo arcu, facilisis vitae tempus a, porta ut dolor. Nam diam tortor, tempor eget semper at, viverra at magna. Vivamus sit amet ante ante. Phasellus eu eros orci. Duis tortor velit, aliquet ut vestibulum ut, semper a tellus. Aenean eget metus odio, non vestibulum metus. Fusce eleifend lectus nec elit pellentesque sit amet fringilla nulla accumsan. Aliquam tempor dignissim adipiscing. Nullam laoreet rhoncus justo nec suscipit. Nullam sollicitudin semper massa, quis mattis tortor consectetur euismod.',
    ),
);

foreach($articles as $id => $article) {
    $datetime = new DateTime($article['timestamp']);

    echo '<article>';
    echo '  <h1>' . $article['title'] . '</h1>';
    echo '  <p>' . getExcerpt($article['text']) . '</p>';
    echo '  <footer>' . $datetime->format('d-m-Y H:i:s') . '</footer>';
    echo '</article>';
}