Description:
------------
Within a function, if an array is passed in by reference, operations on the array get slow within the function as the array grows.  In this example, is_array() gets much slower.  Here is output from the example script with and with out the array variable being passed as reference.

walk_array
10000 0.039294004440308
20000 0.031054973602295
30000 0.026264190673828
40000 0.025592803955078
50000 0.026119947433472
60000 0.025648832321167
70000 0.026489973068237
80000 0.023653984069824
90000 0.028777122497559
walk_array_by_reference
10000 1.7504909038544
20000 6.3624141216278
30000 14.600469827652
40000 28.24251294136
50000 37.910901069641
60000 46.824372053146
70000 60.680932044983
80000 71.826647043228
90000 78.522189855576

Tested against 'PHP 5.5.14-2+deb.sury.org~precise+1 (cli)' and 'PHP 5.6.2 (cli)'

Test script:
---------------
<?php

define('LOOP_COUNT', 100000);
define('LOOP_PRINT', 10000);

$array = [];

walk_array($array);
walk_array_by_reference($array);

function walk_array($array)
{
	echo 'walk_array' . PHP_EOL;

	$timer = microtime(true);

	for ($i = 1; $i < LOOP_COUNT; $i++)
	{
		if ($i % LOOP_PRINT == 0)
		{
			echo $i . ' ' . (microtime(true) - $timer) . PHP_EOL;
			$timer = microtime(true);
		}

		$array[$i] = 1;

		if (is_array($array)) {}
	}
}

function walk_array_by_reference(&$array)
{
	echo 'walk_array_by_reference' . PHP_EOL;

	$timer = microtime(true);

	for ($i = 1; $i < LOOP_COUNT; $i++)
	{
		if ($i % LOOP_PRINT == 0)
		{
			echo $i . ' ' . (microtime(true) - $timer) . PHP_EOL;
			$timer = microtime(true);
		}

		$array[$i] = 1;

		if (is_array($array)) {}
	}
}