Changing debian/rules == failure

[schnitzel meister <schnitzelmeister@gmail.com>]

Hello.

I was under the impression that rebuilding a debian package was a
simple matter of editing the debian/rules file and running debuild.

It fails. Can anyone point me in the right direction?

I need to rebuild php4-sybase to use the Sybase libraries rather than
the tds libraries.
Editing debian/rules seems to break other patches needed by the build.

Here's what I did:
 apt-get install devscripts
 apt-get source php4-sybase
 apt-get build-dep php4-sybase

Next I edited debian/rules so that
 --with-sybase-ct=shared,/usr
was changed to
 --with-sybase-ct=shared,/opt/sybase/OCS_12-5

(I've ignored the --without-sybase-ct lines for moment, since I want
to get it building first.)

Running "debuild -us -uc" gives me an error:
 make: *** [patch-stamp] Error 1
 debuild: fatal error at line 765:
 dpkg-buildpackage failed!

I get errors further up too, such as:
patching file ext/standard/url.c
patching file ext/standard/url_scanner_ex.c
 Hunk #1 FAILED at 1.
 Hunk #2 succeeded at 107 (offset 2 lines).
 Hunk #3 FAILED at 153.
 Hunk #4 succeeded at 353 (offset 13 lines).
 Hunk #5 FAILED at 397.
 Hunk #6 FAILED at 462.
 Hunk #7 succeeded at 498 (offset 26 lines).
 Hunk #8 FAILED at 561.
 Hunk #9 succeeded at 596 (offset 35 lines).
 Hunk #10 FAILED at 643.
 Hunk #11 succeeded at 669 (offset 41 lines).
 Hunk #12 FAILED at 719.
 Hunk #13 succeeded at 758 (offset 48 lines).
 Hunk #14 FAILED at 823.
 Hunk #15 FAILED at 870.
 Hunk #16 succeeded at 1085 (offset 65 lines). 9 out of 16 hunks
FAILED -- saving rejects to file ext/standard/url_scanner_ex.c.rej

The compile works fine before I modify the rules file, and I can't see
any references to sybase within url_scanner_ex.c

Anyone help?