if (is_file('www/htdocs/images/$fullname'))
PHP doesn't do variable substitution inside single quotes. See here.
Here's another way to do it:
$part_name = '05-2416';
$dir = 'www/htdocs/images';
$dp = dir($dir);
while (($file = $dp->read()) !== false) {
if (strpos($file, $part_name) === 0) {
echo $file . '<br />';
}
}