How to access variable of enclosing function?

Question

I'm using a callback function inside an other function and I need to access a variable from this enclosing function but don't know how to do that. Here is an example:

function outer($flag)
{
    $values = array(1, 5, 3, 9);

    usort($values, function($a, $b)
    {
        if ($flag)
        {
            // Sort values in some way
        }
        else
        {
            // Sort values in some other way
        }
    });
}

So I pass some flag to the outer function which is then used inside the sort callback function to decide how to sort the values. Yes, I know I could check the flag in the outer function instead and then call different sort functions but this is not the question.

The question is simply how can I access a variable (or parameter) of the outer function inside the callback. Using a global variable instead is not an option. A "This is not possible" answer is also acceptable if there is really no way to do it.

chiborg · Accepted Answer · 2011-04-21T10:01:52.960

5

There is the use keyword for that. It makes the current value of the variable available in the function.

function outer($flag)
{
  $values = array(1, 5, 3, 9);

  usort($values, function($a, $b) use ($flag)
  {
    if ($flag)
    {
        // Sort values in some way
    }
    else
    {
        // Sort values in some other way
    }
   });
}

edited Apr 21 '11 at 10:01

answered Apr 21 '11 at 09:43

chiborg

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I really like the answer from knittl - use two functions, depending on the flag. – chiborg Apr 21 '11 at 09:50
That was new to me. Thanks, works great! BTW: It must be `use ($flag)` and not `use $flag`. – kayahr Apr 21 '11 at 09:56

score 3 · Answer 2 · edited May 23 '17 at 12:08

there are ~~two~~ three ways:

use global, which is not an option for you
call different callbacks, depending on $flag
see chiborg's answer for how to do this properly with php's new lambda functions/inline callbacks -- performance overhead mentioned below will still apply though

i would go for option 2. because, there will also be a performance improvement: when doing 10000 comparisons, your flag will be checked 10000 times. when using two separate callback functions, the flag will be checked only once instead (comparison probably being the operation used most in sorting algorithms).

score 0 · Answer 3 · answered Apr 21 '11 at 09:41

0

This is not possible without explicit passing the variable.

You have to pass $flat to the function() like function($a, $b, $flag).

answered Apr 21 '11 at 09:41

tamasgal

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How to access variable of enclosing function?

3 Answers3