Flatten a multidimensional array

function flatten_multidimensional_array($array, $parentKey = NULL, $output = NULL )
{
	$parentKey 	= isset($parentKey) ? $parentKey.'_' : '';
	$output 	= isset($output) ? $output : array();
	
	foreach ($array as $key => $value)
	{
		switch (gettype($value))
		{
		    case "string":
		        $output[$parentKey.$key] = $value;
		        break;
		    default:
		    	$output = array_merge($output, flatten_multidimensional_array($value, $parentKey.$key, $output));
		}
	}
	return $output;
}

$output = flatten_multidimensional_array($array);