PHP中相对路径和锚文件的绝对路径

当我有一个相对路径和另一个应该作为锚点的绝对路径时，如何计算文件的绝对路径。

例如：

$anchor = '/tmp/uploads/abcd1234/import.xml';
$relative = 'main_image.png';
$abs = foo($anchor, $relative); # fn `foo` is what I'm looking for
assert($abs == '/tmp/uploads/abcd1234/main_image.png');

或者稍微复杂一点：

$anchor = '/tmp/uploads/abcd1234/import.xml';
$relative = '../resource/dummy.jpg';
$abs = foo($anchor, $relative); # fn `foo` is what I'm looking for
assert($abs == '/tmp/uploads/resource/dummy.jpg');