Empty array after passing array by reference to array_walk()

Question

I have made this little class:

class Analyzer {
    public static function analyze($phrases) {
        $sortedPhrases = array();
        array_walk($phrases, array('self', 'splitByLength'), $sortedPhrases);
        var_dump($sortedPhrases);
    }

    private static function splitByLength($item, $key, &$resArr) {
        // line stolen from here: http://stackoverflow.com/a/4786840/603003
        // thanks to arnaud576875 <http://stackoverflow.com/users/576875/arnaud576875>
        $len = count( preg_split('#\PL+#u', $item, -1, PREG_SPLIT_NO_EMPTY) );
        if (!isset($resArr[$len])) {
            $resArr[$len] = array();
        }
        $resArr[$len][] = $item;

        var_dump($resArr);
    }
}

$phrases = array(
    "I can't believe the great content",
    "I can't understand the superior information",
    "I can't comprehend the amazing data",
    "I cannot analyze the amazing data",
    "I haven't written to the amazing data"
);
Analyzer::analyze($phrases);

Executing the script results in the output below:

array (size=1)
  7 => 
    array (size=1)
      0 => string 'I can't believe the great content' (length=33)

...

array (size=3)
  7 => 
    array (size=3)
      0 => string 'I can't believe the great content' (length=33)
      1 => string 'I can't understand the superior information' (length=43)
      2 => string 'I can't comprehend the amazing data' (length=35)
  6 => 
    array (size=1)
      0 => string 'I cannot analyze the amazing data' (length=33)
  8 => 
    array (size=1)
      0 => string 'I haven't written to the amazing data' (length=37)

array (size=0)
  empty

All outputs are actually correct except the last one which comes from Analyzer::analyze(). It seems that the variable $sortedPhrases is somehow cleared after array_walk().

Answer 1

Take a better look at array_walk's documentation page.

userdata

If the optional userdata parameter is supplied, it will be passed as the third parameter to the callback funcname.

That's the third parameter. It's not a reference, it's just a value that gets passed to your callback function.

---

One (of many) solutions to your problem is to use an object instead (objects are always passed by reference):

class Analyzer {
    public static function analyze($phrases) {
        $arrObj = new ArrayObject();
        array_walk($phrases, array('self', 'splitByLength'), $arrObj);
        var_dump($arrObj->getArrayCopy());
    }

    private static function splitByLength($item, $key, $arrObj) {
        // line stolen from here: http://stackoverflow.com/a/4786840/603003
        // thanks to arnaud576875 <http://stackoverflow.com/users/576875/arnaud576875>
        $len = count( preg_split('#\PL+#u', $item, -1, PREG_SPLIT_NO_EMPTY) );
        if (!isset($arrObj[$len])) {
            $arrObj[$len] = array();
        }
        $arrObj[$len][] = $item;

        var_dump($arrObj->getArrayCopy());
    }
}

(it doesn't have to be an ArrayObject, it can be a stdClass object with an array property, or create your own class if you want...)

---

Or you can wrap your call within an anonymous function if you really want to work with a reference:

$static = get_called_class();
array_walk($phrases, function($item, $key) use($static, &$sortedPhrases){
  $static::splitByLength($item, $key, $sortedPhrases);
});