What is a callback function and how do I use it with OOP

Question

I want to use the php simple HTML DOM parser to grab the image, title, date, and description from each article on a page full of articles. When looking at the API I notice it has a set_callback which Sets a callback function. However im not sure what this does or how I would use it? In one of the examples its used to call a function which strips out some stuff, im wondering if you have to use this to call all functions?

I guess im wondering why I use this, and what does it do as I have never come across a callback function before!

Petula

Answer 1

Here's a basic callback function example:

<?php

function thisFuncTakesACallback($callbackFunc)
{
    echo "I'm going to call $callbackFunc!<br />";
    $callbackFunc();
}

function thisFuncGetsCalled()
{
    echo "I'm a callback function!<br />";
}

thisFuncTakesACallback( 'thisFuncGetsCalled' );
?>

You can call a function that has its name stored in a variable like this: $variable().

So, in the above example, we pass the name of the thisFuncGetsCalled function to thisFuncTakesACallback() which then calls the function passed in.

Seasickness

Great and simple answer, thanks. I've one question though, what is the minimum required PHP version that has this feature available? – Gamo

This answer is out-of-date. Today, PHP uses the term 'callback' to refer to a specific kind of function. For more information read PHP's own documentation on the matter: php.net/manual/en/function.call-user-func.php – Jolie

Answer 2

A callback function will use that function on whatever data is returned by a particular method.

I'm not sure how this particular library works, but it could be something as simple as:

$html = file_get_html('http://example.com');
$html->set_callback('make_bold');
$html->find('#title'); // returns an array

function make_bold($results) {
// make the first result bold
  return '<b>'.$results[0].'</b>';
}

ie, The function "make_bold()" will be run on any data found. Again, I'm not sure how this particular library works (ie, what methods the callback function will get called on)

Guileless

Answer 3

A callback is either a function, an object instance' method, or a static method on a class. Either way, it's kind of a function pointer. In some languages, functions are a specific type. So you could assign a function to a variable. These are generally called function oriented languages. A good example is Javascript.

In PHP, a callback can be any of:

$fn = 'foo'; // => foo()
$fn = array($obj, 'foo'); // => $obj->foo()
$fn = array('Foo', 'bar'); // => Foo::bar()

See the manual entry for is_callable.

You can invoke a callback with the rather verbose function call_user_func.

Groveman

Answer 4

Defination

A callbacks/callable is a simple function(either it is anonymous or named function) that we pass to another function as function parameter which in the result returns that passed function.

Example

function iWillReturnCallback($callBackHere){
    return $callBackHere;
}

function iAmCallBack(){
    echo "I am returned with the help of another function";
}

iWillReturnCallback(iAmCallBack());

//--Output -> I am returned with the help of another function

Don't be confused

There are some default functions in php that accepts the name of the callback function as a string in their parameter because of avoiding conflicting between the constant name and function name. So don't be confused in these kind of things.

Antabuse

Answer 5

With PHP 5.3, you can now do this:

function doIt($callback) { $callback(); }

doIt(function() {
    // this will be done
});

Finally, a nice way to do it. A great addition to PHP, because callbacks are awesome.

Kaleykaleyard

What happens if $callback is not a function? Does PHP throw a fatal error, warning, or nothing? – Praenomen

Answer 6

The aim is to call a function that we want eg: secretCode() but we want to use another function as a helper or service to call it for us:

<?php
    
    // $call parameter can be anything
    function callBackServiceCenter($call)
    {
        echo "[callBackServiceCenter]: Hey, this is callBackServiceCenter function <br>We have received your command to call your requested function and we are now calling it for you! <br />";
        // Below is the part where it will call our secretCode()'s function
        $call();
        // And we can print other things after the secretCode()'s function has been executed:
        echo "[callBackServiceCenter]: Thank you for using our service at callBackServiceCenter. Have a nice day!<br />";
    }
    
    function secretCode()
    {
        echo "[secretCode]: Hey, this is secretCode function. Your secret code is 12345<br />";
    }
    
    callBackServiceCenter( 'secretCode' );
?>

Output:

[callBackServiceCenter]: Hey, this is callBackServiceCenter function
We have received your command to call your requested function and we are now calling it for you!
[secretCode]: Hey, this is secretCode function. Your secret code is 12345
[callBackServiceCenter]: Thank you for using our service at callBackServiceCenter. Have a nice day!

Socher