Description:
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(Request for reopening bug #35765)
Sorry to restart this thread more than 3 years later, but I'm facing the very same problem and cannot add a comment on a "won't fix" bug.

The problem concerns the fopen()'s ftp wrapper incorrectly relying on the ftp SIZE command to check whether a file is downloadable.

I do think that's not a correct behaviour.
For example I'm using a ftp server from a well-known affiliate marketing company, that's serving "virtual" files, that are not displayed in the ftp server as such, but downloadable by any ftp client I could try... except php's fopen wrapper.

The problem is that this server returns a "550 Not a plain file" error for a SIZE request. This is not a plain file, but it is perfectly downloadable.

The SIZE command is *not* standardized in RFC 959. It may be used by ftp clients, but only to try to get some information about the file.

Refusing to download a file due to a SIZE command failing is not RFC compliant, while the servers the previous bug reporter and I are using, are.

Reproduce code:
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$fp = fopen("ftp://example.com/test.xml.gz", "rb");
var_dump($fp);

Expected result:
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bool(true)

Actual result:
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Warning: fopen(ftp://example.com/test.xml.gz) [function.fopen]: failed to open stream: FTP server reports 550 /test.xml.gz: not a plain file.
bool(false)