What operating system and more specifically what shell version are you using?

//Mikael

tried on:
GNU bash, version 3.1.5(1)-release (i686-pc-linux-gnu)
GNU bash, version 3.1.17(1)-release (i686-pc-linux-gnu)

Haven't been able to reproduce this problem on either

 GNU bash, version 3.00.15(1)-release(x86_64-redhat-linux-gnu)
 GNU bash, version 3.1.7(2)-release (i586-mandriva-linux-gnu)

With PHP 5.0.5 or 4.4.1

Would you mind checking what "/bin/sh --version", "aclocal --version" and "autoconf --version" outputs and attach an excerpt from your generated configure script around line 1376 where the error occur.

Also; please check where your zlib installation resides (for example using "locate zlib.h", most likely it's in /usr/include/zlib.h or something similar)

//Mikael

zlib path: 
/usr/include/zlib.h

versions:
GNU bash, version 3.1.17(1)-release (i686-pc-linux-gnu)
aclocal (GNU automake) 1.9.6
autoconf (GNU Autoconf) 2.59

code:
  if test "$PHP_ZLIB_DIR" != "no" && test "$PHP_ZLIB_DIR" != "yes"; then
    if test -f "$PHP_ZLIB_DIR/include/zlib/zlib.h"; then
      PHP_ZLIB_DIR="$PHP_ZLIB_DIR"
      PHP_ZLIB_INCDIR="$PHP_ZLIB_DIR/include/zlib"
    elif test -f "$PHP_ZLIB_DIR/include/zlib.h"; then
      PHP_ZLIB_DIR="$PHP_ZLIB_DIR"
      PHP_ZLIB_INCDIR="$PHP_ZLIB_DIR/include"
        else
      { echo "configure: error: Can't find ZLIB headers under "$PHP_ZLIB_DIR"" 1>&2; exit 1; }
    fi
  else
    for i in /usr/local /usr; do
      if test -f "$i/include/zlib/zlib.h"; then
        PHP_ZLIB_DIR="$i"
        PHP_ZLIB_INCDIR="$i/include/zlib"
      elif test -f "$i/include/zlib.h"; then
        PHP_ZLIB_DIR="$i"
        PHP_ZLIB_INCDIR="$i/include"
      fi
    done
  fi

this syntax error appeared on redhat 8.0 too
memcache verison 2.0.4

Reproduce code:
---------------
checking for gawk... gawk
: command not found
checking whether to enable memcache support... yes, shared
: command not found
: command not found
./configure: line 1376: syntax error near unexpected token `elif'
'/configure: line 1376: `    elif test -f "$PHP_ZLIB_DIR/include/zlib.h"; then

I cannot reproduce it neither on Redhat nor on Suse.
There is absolutely nothing special and there are tons of "elif" in other .m4 files all around PHP sources, so I honestly do not see a reason for such code to fail except for your misconfiguration.

Please create a shell script like this:
#!/bin/sh

    if test -f "$PHP_ZLIB_DIR/include/zlib/zlib.h"; then
      PHP_ZLIB_DIR="$PHP_ZLIB_DIR"
      PHP_ZLIB_INCDIR="$PHP_ZLIB_DIR/include/zlib"
    elif test -f "$PHP_ZLIB_DIR/include/zlib.h"; then
      PHP_ZLIB_DIR="$PHP_ZLIB_DIR"
      PHP_ZLIB_INCDIR="$PHP_ZLIB_DIR/include"
    fi

And try to execute it.

I'm experiencing the same problem om Debian Sarge.

Output is:
checking for gawk... gawk
: command not foundache-2.0.4/configure: line 2528:
checking whether to enable memcache support... yes, shared
: command not foundache-2.0.4/configure: line 2577:
: command not foundache-2.0.4/configure: line 2578:
/tmp/tmpTTJ1L4/memcache-2.0.4/configure: line 2612: syntax error near unexpected token `elif'
/tmp/tmpTTJ1L4/memcache-2.0.4/configure: line 2612: `    elif test -f "$PHP_ZLIB_DIR/include/zlib.h'; then
ERROR: `/tmp/tmpTTJ1L4/memcache-2.0.4/configure' failed

Running dos2unix, to remove ^M characters from the config.m4 files allows a good ./configure

 tar xzf memcache-2.0.4.tgz && cd memcache-2.0.4 && dos2unix *m4 && phpize && ./configure

Wow that was strange..

I had the same problem.. I'm running multiple boxes, all with Debian Sarge, all very up to date and 2 of them had no problem running ./configure

The last one however, did.. Couldn't convert at all.. only after removing the windows linebreaks it did

How can this be? The boxes are using the exact same packages and are nearly identical.

This was a problem with ^M chars in the config.m4 file, closing bug