Description:
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Given the following scenario:

open_basedir enabled to /var/www/foo
upload_tmp_dir set to /var/www/foo/tmpdir
No free file space to handle the upload in the temp dir specified.

Causes PHP to throw an error that doesn't make a lot of sense to my end users:

Warning: Unknown: open_basedir restriction in effect. File(/tmp) is not within the allowed path(s): (/var/www/foo/tmpdir) in Unknown on line 0

This makes things look like a permissions issue.  I have tracked down the source of the error to main/php_open_temporary_file.c:254  There is a comment there that if the first attempt fails, it will use the system temp dir.  While that might be fine on systems without open_basedir enabled, it can cause some confusing error messages otherwise.  The error message of "File upload error - unable to create a temporary file" is perfect for this error, and it does show up.  It just isn't the first or most noticeable error.

I notice that in the php_get_temporary_directory function (which is what determines /tmp as the temp dir) that there are options for changing this default.  I plan on setting TMPDIR per vhost as a work around, but this seems unnecessary as I already set upload_tmp_dir.

I'm not sure what the perfect answer to this is, but I'm not sure I agree with magically changing something I specifically set without telling me.  Some of the extremists might even label this a security issue.