如何将数组作为可选参数传递并迭代它？

I am trying to modify phpgraphlib so that I can generate a legend when multiple bar colors are used. I added an array with colors as optional parameter in generateLegend() but it doesn't seem to work. I don't know what's wrong. I have no prior experience with PHP, but it seemed to me passing an array as optional parameter must be possible Here is my code:

protected function generateLegend(array $colors = array())
{
    // here is some code        
    if($this->bool_multi_color_bars) {
        // gets here
        if (!empty($colors)) {
            // doesn't get here
            $index = 0;
            foreach($colors as $key => $item) {
                // here is some code that creates the colored boxes
                $index++;
            }
        }
    }
}

And here is the code that calls the function:

$colors = array();
foreach($data as $key => $value) {
    if($value < 3) {
        $colors[$key] = 'green';
    }
    elseif($value < 8) {
        $colors[$key] = 'orange';
    }
    else {
        $colors[$key] = 'red';
    }
}
$graph->setBarColors($colors);
$graph->setLegend(true, $colors);
$graph->createGraph();

EDIT: generateLegend() is called with the follwing code:

if ($this->bool_legend) { 
            $this->generateLegend(); 
        }

For the sake of readability I left most of the code out, but I can see that the method is called (therefore I added the comments where the code does get and not)

I'm not sure, what you actually want. What you currently see is the expected behavior of empty. An array without elements is empty.

var_dump(empty([])); // true

If you want to test, if the optional param was actually set, you could use func_num_args.

if (func_num_args() > 0) {
    // $colors was set
}

Or use an other default argument and test against the type.

function foo(array $bar = null) {
    if ($bar === null) {
        // ...
    }
}