Description:
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I'd created a fake email code fragment:
  e-mail: foo.<?php echo base_convert(time(),10,36).".".base_convert(ip2long($_SERVER['REMOTE_ADDR']),10,36) ?>@mydomain.com

However, long2ip(base_convert("cxcapb",36,10)) gave the wrong answer (should be 209.105.205.209, got 46.150.50.47)

It turns out that base_convert(...,10,36) should have returned -cxcapb instead of cxcapb, since ip2long("209.105.205.209") = -781595183 and ip2long("46.150.50.47") = 781595183.

At this point, I'm not sure if there's legions of programs that work around this bug and would be broken by a fix.

Test script:
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$a=base_convert(-781595183,10,36);
if (base_convert($a,36,10) != -781595183) { echo "fail\n"; } else { echo "success\!"; }


Expected result:
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success!

Actual result:
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fail