When was + treated as space in the path part of the URL? Sure it's been treated as space in the query part, but that would be a weird breaking change if early web treated path and query the same way, and then later standards made them different.
At the time S3 launched the URL spec was RFC 1738 and we had HTML 4.01[2]. And, the URI syntax (all the way back in 1998) noted to use %20 for a space[3].
As far as I can tell, this traces its history back to encoding for forms[4]. It's been used far beyond the encoding for forms and maybe someone can explain why.
It's also not just PHP whose function is that way. In Python urlencode encodes as a + (at least in 2.7.x).
I remember working on the web many years ago where "+" is what was used. This may have been a spec misinterpretation or something else. In any case, it was common enough.
Note, I'm not saying it was right. Just not uncommon.
