Using Callback Functions in PHP

A commonly used technique when programming is to use callbacks. A callback is a piece of code that is passed to a function that is later executed by that function. In this article I will show you how to define and call callbacks in PHP, as well as a practical example of how callbacks can be useful.

To demonstrate how callbacks can be beneficial, assume we have a PHP script that when called will download an RSS feed from 10 different web sites. If it takes 2 seconds to download from each site, it will take 20 seconds to download from all sites.

The code to achieve this may look something like the following code. I’ve added a called to sleep() to demonstrate how slow this script may be.

$site,
‘data’ => ‘downloaded data here’
);
}

return $ret;
}

// imaginary list of sites to download from
$sites = array(
‘http://www.example1.com’,
‘http://www.example2.com’
// more sites…
);

// start downloading and store the return data
$data = downloadFromSites($sites);

// output the return data
foreach ($data as $row) {
echo sprintf(“Finished downloading from %s\n”, $row[‘site’]);
}
?>