Your comments

whoops, that's why I asked to make sure, restoring to normal right away, sorry o_O
thanks for the pointer! curl needs the longest code to execute, and that's the reason I wasn't using it... but surprisingly enough it can be up to 5 times faster than both flocse(fopen()) and get_headers() (~0.07 seconds instead of up to 0.32seconds)
I actually timed the fclose(fopen()) procedure, and I get from 0.2s to 1s regardless of the remote file (Tested on some 13MB wikimedia picture :D), so I suppose I am not getting the whole file anyway.
But to be on the safe side, to reduce the php exution time I'll test if cURL terminates faster.