How to load multiple values from a field with views_php module?
As Syd said, it seems like you need a foreach loop. It's not readily apparent what you're wanting to do with the $source variable, but I'm guessing it's something like the following:

foreach ($data-&gt;_field_data['nid']['entity']-&gt;my_field['und'] as $a_file) {
    echo '&lt;img src="' . $a_file['filename'] . '"&gt;';
}