Are you compiling against libmemcached-0.38?

yes that is correct...

It's a known problem. Libmemcached-0.38 broke binary compatibility. I'll try to make a release that fixes this soon.

I apologize for not being aware of this existing issue.  Thank you again.

I can confirm the same compilation problem on Ubuntu 8.04.4 LTS 64-bit.

Thank you for your bug report. This issue has been fixed
in the latest released version of the package, which you can download at
http://pecl.php.net/get/memcached

Try 1.0.1 release.

I was able to compile this in centos 5.14 32bit with version 5.2.13 and libmemcached 0.38.  However in 5.14 64bit and 5.2.13 and libmemcached 0.38 I get this:

/bin/sh /root/memcached-1.0.1/libtool --mode=compile cc -I/usr/local/include/php  -I. -I/root/memcached-1.0.1 -DPHP_ATOM_INC -I/root/memcached-1.0.1/include -I/root/memcached-1.0.1/main -I/root/memcached-1.0.1 -I/usr/local/include/php -I/usr/local/include/php/main -I/usr/local/include/php/TSRM -I/usr/local/include/php/Zend -I/usr/local/include/php/ext -I/usr/local/include/php/ext/date/lib -I/usr/local/include  -DHAVE_CONFIG_H  -m64   -c /root/memcached-1.0.1/php_memcached.c -o php_memcached.lo
mkdir .libs
 cc -I/usr/local/include/php -I. -I/root/memcached-1.0.1 -DPHP_ATOM_INC -I/root/memcached-1.0.1/include -I/root/memcached-1.0.1/main -I/root/memcached-1.0.1 -I/usr/local/include/php -I/usr/local/include/php/main -I/usr/local/include/php/TSRM -I/usr/local/include/php/Zend -I/usr/local/include/php/ext -I/usr/local/include/php/ext/date/lib -I/usr/local/include -DHAVE_CONFIG_H -m64 -c /root/memcached-1.0.1/php_memcached.c  -fPIC -DPIC -o .libs/php_memcached.o
In file included from /usr/local/include/libmemcached/storage.h:15,
                 from /usr/local/include/libmemcached/memcached.h:51,
                 from /root/memcached-1.0.1/php_memcached.c:42:
/usr/local/include/libmemcached/libmemcached/memcached.h:223: error: conflicting types for ?memcached_set_memory_allocators?
/usr/local/include/libmemcached/allocators.h:25: error: previous declaration of ?memcached_set_memory_allocators? was here
/usr/local/include/libmemcached/libmemcached/memcached.h:230: error: conflicting types for ?memcached_get_memory_allocators?
/usr/local/include/libmemcached/allocators.h:32: error: previous declaration of ?memcached_get_memory_allocators? was here
In file included from /root/memcached-1.0.1/php_memcached.c:42:
/usr/local/include/libmemcached/memcached.h:56: error: redefinition of ?struct memcached_st?
/usr/local/include/libmemcached/memcached.h:144: error: conflicting types for ?memcached_clone?
/usr/local/include/libmemcached/libmemcached/memcached.h:122: error: previous declaration of ?memcached_clone? was here
/usr/local/include/libmemcached/memcached.h:147: error: conflicting types for ?memcached_get_user_data?
/usr/local/include/libmemcached/libmemcached/memcached.h:233: error: previous declaration of ?memcached_get_user_data? was here
/root/memcached-1.0.1/php_memcached.c: In function ?php_memc_get_impl?:
/root/memcached-1.0.1/php_memcached.c:369: warning: passing argument 4 of ?memcached_mget_by_key? from incompatible pointer type
/root/memcached-1.0.1/php_memcached.c:406: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c:438: warning: passing argument 4 of ?memcached_mget_by_key? from incompatible pointer type
/root/memcached-1.0.1/php_memcached.c: In function ?php_memc_getMulti_impl?:
/root/memcached-1.0.1/php_memcached.c:575: warning: passing argument 4 of ?memcached_mget_by_key? from incompatible pointer type
/root/memcached-1.0.1/php_memcached.c:605: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c:608: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c: In function ?php_memc_getDelayed_impl?:
/root/memcached-1.0.1/php_memcached.c:724: warning: passing argument 4 of ?memcached_mget_by_key? from incompatible pointer type
/root/memcached-1.0.1/php_memcached.c: In function ?zim_Memcached_fetch?:
/root/memcached-1.0.1/php_memcached.c:799: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c:802: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c: In function ?zim_Memcached_fetchAll?:
/root/memcached-1.0.1/php_memcached.c:850: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c:853: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c: In function ?zim_Memcached_getResultMessage?:
/root/memcached-1.0.1/php_memcached.c:1868: warning: initialization discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c: In function ?php_memc_do_result_callback?:
/root/memcached-1.0.1/php_memcached.c:2328: warning: assignment discards qualifiers from pointer target type
/root/memcached-1.0.1/php_memcached.c:2331: warning: assignment discards qualifiers from pointer target type
make: *** [php_memcached.lo] Error 1

Sorry, but your problem does not imply a bug in PECL itself.  For a
list of more appropriate places to ask for help using PECL, please
visit http://pecl.php.net/support.php as this bug system is not the
appropriate forum for asking support questions. 

Thank you for your interest in PECL.

This has nothing to do with pecl/memcached. It looks like you have both 0.37 and 0.38 installed at once and they are conflicting.