Question

    function heaviside(&$value, $key, &$array)
    {
            if($key > 0)
            $value = $array[$key-1].$array[$key];
    }

    function test_heaviside()
    {
            for($i=0; $i<10; $i++)
            {
                    $array[$i] = $i;
            }
            array_walk($array, 'heaviside', &$array);
            print_r($array);
    }

    test_heaviside();

My problem is that the above code will generate this warning:

PHP Warning: Call-time
pass-by-reference has been deprecated
– argument passed by value; If you would like to pass it by reference,
modify the declaration of
array_walk(). If you would like to
enable call-time pass-by-reference,
you can set
allow_call_time_pass_reference to true
in your INI file. However, future
versions may not support this any
longer.

And if I remove & in &$array in my call to array_walk, this function will not return this correct result. In the first case, where it works, it returns this result:

[0] => 0
[1] => 01
[2] => 012
[3] => 0123
[4] => 01234
[5] => 012345
[6] => 0123456
[7] => 01234567
[8] => 012345678
[9] => 0123456789

Whereas if I remove & it returns:

[0] => 0
[1] => 01
[2] => 12
[3] => 23
[4] => 34
[5] => 45
[6] => 56
[7] => 67
[8] => 78
[9] => 89

I need help understanding this or simply to find a solution other than changing .ini.

Answer 1

You’re abusing array_walk here — your callback function isn’t actually returning the new value. array_walk is intended to work with one and only one value from the target array, and can not work with anything by reference.

You can achieve the effect you’re looking for by using a simple for loop:

// Set up the array.
    $array = array();
    for($i = 0; $i < 10; $i++)
        $array[$i] = $i;
// Process it, starting at the second element
    for($i = 1; $i < count($array); $i++)
        $array[$i] = $array[$i - 1] . $array[$i];
    print_r($array);