Callback functions

edwardp

Can anyone point me to a decent site that explains about callback functions?

laserlight

In PHP, a callback function is simply one invoked by providing its name as the argument of another function, rather than by calling it directly.

edwardp

Can you pass parameters to callback functions?

bubblenut

Depends on the function it's being called from. Some have a fixed length parameter list which your function must comply to, others have variable parameter lists which you can add to. What exactly are you wanting to do?

edwardp

i've just been looking at script examples and a few use this callback function and im trying to find out about it

bubblenut

code?

laserlight

<?php
function foo($bar, $baz) {
	return $bar . ' loves ' . $baz;
}

echo call_user_func('foo', 'boy', 'girl');
$str = call_user_func('foo', 'edwardp', 'bubblenut');
echo '<br>heh, ' . $str;
?>

In this example with [man]call_user_func/man, the callback foo() is invoked by passing its name, 'foo', as the first argument.
The next two arguments of call_user_func() are passed to foo() as its own arguments.

section31

I am so confused about this...

Can someone please explain this example for me.

<?php
function cmp($a, $b)
{
   if ($a == $b) {
       return 0;
   }
   return ($a < $b) ? -1 : 1;
}

$a = array(3, 2, 5, 6, 1);

usort($a, "cmp");

?>

I even echo'd out $a and $b in the function and it still didn't make sense.

Weedpacket

As bubblenut says, it depends on the function it is being called from.

[man]usort[/man]
The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.

So usort() is giving the comparison function pairs of values from the array whenever it needs to know which of them is larger.

section31

Yeah, but what I don't understand is what the second parameter $b is in the function.

What algorithm is it using to pass the paramaters. when I echo these out in the function. I get this.

a - 5 | b - 2
a - 5 | b - 3
a - 5 | b - 6
a - 1 | b - 5
a - 2 | b - 1
a - 3 | b - 2

I understand that in order to use this function... you don't need to understand how it passes what values...but I would like to know anyway.

Weedpacket

Originally posted by section31
Yeah, but what I don't understand is what the second parameter $b is in the function.

Same sort of thing as $a - an element of the array. After all, the comparison function is supposed to compare two elements of the array to decide which of them is larger.

What algorithm is it using to pass the paramaters. when I echo these out in the function. I get this.

My guess would be quicksort; that's pretty much the standard in C libraries (the fact that the sort is unstable is a clue).

http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?query=quicksort

Actually, why guess? All the information (i.e., the source code) is publically available. Yes, it does use quicksort, but an internal implementation (cf. array.c, zend_qsort.c).

section31

Thank you, i now understand and I must say that this function is mighty useful. Especially with complex multidimensional arrays.