When compiling PHP on MacOSX 10.1 you must update line 1457 
of the ltconfig file. Trying to compile the source code 
from php.net as is results in this error:

/usr/bin/ld: -undefined error must be used when -
twolevel_namespace is in effect 
make[1]: *** [libphp4.la] Error 1 
make: *** [all-recursive] Error 1 


Original source line# 1457 of ltconfig downloaded from 
php.net:
archive_cmds='$CC -bundle -undefined suppress -o $lib 
$libobjs $deplibs $linkopts'

What must be used on line 1457 of ltconfig to successfully 
compile:
archive_cmds='$CC -force_flat_namespace -bundle -undefined 
suppress -o $lib $libobjs $deplibs $linkopts' 


It took me a long time to find the answer for this problem. 
There is an article on the Apple developer pages that gave 
me the change, but they have the wrong line number listed 
[maybe the source has been updated]

Article that told me how to fix it:
http://developer.apple.com/internet/macosx/php.html