Performance issues with array testing in PHP. Why?

Question

Fast code (100ms):

// Mixed could theoretically be an object or an array. 
// In this example it's an array.
$mixed = array();
for($i = 0; $i < 10000; $i++) {
  if (is_array($mixed)) {
    $mixed[getRandomStr(15)] = 'abcd';
  } else {
    // Do something else which we don't care about here
  }
}

Terribly slow code (4000ms): (note that $mixed is passed by reference)

function put(&$mixed, $key, $value) {
  if (is_array($mixed)) {
    $mixed[$key] = $value;
  } else {
    // Do something else which we don't care about here
  }
}

$mixed = array();
for($i = 0; $i < 10000; $i++) {
  put($mixed, getRandomStr(15), 'abcd');
}

Now it's fast again: (note that the is_array check has been dropped here)

function put(&$mixed, $key, $value) {
  $mixed[$key] = $value;
}

$mixed = array();
for($i = 0; $i < 10000; $i++) {
  put($mixed, getRandomStr(15), 'abcd');
}

Now we wrap the array inside an object and reintroduce the is_array check ... and it's still fast:

function put($objWrap, $key, $value) {
  if (is_array($objWrap->mixed)) {
    $objWrap->mixed[$key] = $value;
  } else {
    // Do something else which we don't care about here
  }
}

$objWrap = new stdClass;
$objWrap->mixed = array();

for($i = 0; $i < 10000; $i++) {
  put($objWrap, getRandomStr(15), 'abcd');
}

This happens with PHP 5.3 and 5.4.

Does anyone know why? (and/or how to work around this problem)

My guess: passing a reference to the array to is_array() causes the whole array to be copied, slowing the process down.

Note: you can replace is_array() with !is_object() or even with gettype() === 'array'. It does not change a bit.