Without wanting to be killed but there are more postings concerning this problem. The killing part is in the fact that other postings are with windows.
I had the same problem on windows. Turns out to be a "double" problem.
1st -- rights: local systemaccount did not have enough priviliges to access network shares. I.e. I now have the apache service run as a account which does have rights for that directory
2nd -- php problem: php does not seem to catch windows drive-letters from about php 4.3 or so. For me it worked to only use UNC coding (\\server\sharename\directory\).
So not one, but two challenges. Might be the in the same range with Linux:
1st -- rights problem
2nd -- use full unc paths.