Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc.

If possible, make the script source available online and provide
an URL to it here. Try to avoid embedding huge scripts into the report.

this crash occurs in the stock packages of php in the versions php5-200504180030, php5-STABLE-200504180037, and php-5.0.4, which were the latest of each at the time of this bug post.

the steps needed to reproduce:   =P

tar -xvjf php5-*
cd php5-<tab>
./configure #i normally use only --enable-maintainer-zts
make && make install

Please try using this CVS snapshot:

  http://snaps.php.net/php5-STABLE-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5.0-win32-latest.zip

exact same problem persists using php5-STABLE-200504251833:

$ ./configure --enable-maintainer-zts --disable-cgi --with-readline --with-ssh2
$ make && make install
(...)
Build complete.
(It is safe to ignore warnings about tempnam and tmpnam).

Installing PHP CLI binary:        /usr/local/bin/
Installing PHP CLI man page:      /usr/local/man/man1/
Installing PEAR environment:      /usr/local/lib/php/
Signal 11
make[1]: *** [install-pear-installer] Error 139
make: *** [install-pear] Error 2

Works fine here.
Please try latest snapshot once more.

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

I have the same behavior with the following snapshot:
php5-STABLE-200505260839

$ ./configure --prefix=/usr/local/stow/php5-STABLE-200505260839 --enable-discard-path --enable-fastcgi

... everything is fine ...

$ make

... everything is fine ...

$ make install
Installing PHP SAPI module:       cgi
Installing PHP CGI into: /usr/local/stow/php5-STABLE-200505260839/bin/
Installing PEAR environment:      /usr/local/stow/php5-STABLE-200505260839/lib/php/
Signal 11
make[1]: *** [install-pear-installer] Error 139
make: *** [install-pear] Error 2

I can add an strace if asked for.

Still can't reproduce.
Try to run this command (in the sources root):
./sapi/cli/php -n -dshort_open_tag=0 -dsafe_mode=0 ./pear/install-pear.php -d /tmp -b "/usr/local/stow/php5-STABLE-200505260839/bin/" ./pear/package-*.xml 
And if it fails - generate GDB backtrace.

Still segfault, here is the backtrace. I'll recompile with debug enabled and if the behavior is the same, I'll send you a more complete backtrace:

#0  0x0c8025ff in ?? ()
#1  0x004d2d2a in _start_element_handler (user=0x1036ede8, name=0x1039f353 "package", attributes=0x103a1bc0)
    at /cygdrive/c/Temp/php5-STABLE-200505260839/ext/xml/compat.c:65
#2  0x009299d8 in xmlParseStartTag () from /usr/bin/cygxml2-2.dll
#3  0x009357fb in xmlParseChunk () from /usr/bin/cygxml2-2.dll
#4  0x004d368a in php_XML_Parse (parser=0x1036ede8, 
    data=0x1036f1b0 "<?xml version=\"1.0\" encoding=\"ISO-8859-1\" ?>\n<!DOCTYPE package SYSTEM \"package.dtd\">\n<package version=\"1.0\">\n  <name>Archive_Tar</name>\n  <summary>Tar file management class</summary>\n  <description>Th"..., 
    data_len=2601, is_final=1) at /cygdrive/c/Temp/php5-STABLE-200505260839/ext/xml/compat.c:512
#5  0x004d1d84 in zif_xml_parse (ht=3, return_value=0x1039f2f8, this_ptr=0x0, return_value_used=1)
    at /cygdrive/c/Temp/php5-STABLE-200505260839/ext/xml/xml.c:1340
#6  0x0052f456 in zend_do_fcall_common_helper (execute_data=0x2276b0, opline=0x1019e198, op_array=0x102c7d08)
    at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:2747
#7  0x0052c139 in execute (op_array=0x102c7d08) at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:1417
#8  0x0052f24d in zend_do_fcall_common_helper (execute_data=0x227a00, opline=0x10185fa0, op_array=0x102c7c20)
    at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:2776
#9  0x0052c139 in execute (op_array=0x102c7c20) at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:1417
#10 0x0052f24d in zend_do_fcall_common_helper (execute_data=0x22b4d0, opline=0x1012c5b8, op_array=0x10104e08)
    at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:2776
#11 0x0052c139 in execute (op_array=0x10104e08) at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:1417
#12 0x0052f24d in zend_do_fcall_common_helper (execute_data=0x22d9b0, opline=0x1009dfd0, op_array=0x1008e1d0)
    at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:2776
#13 0x0052c139 in execute (op_array=0x1008e1d0) at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend_execute.c:1417
#14 0x0050d485 in zend_execute_scripts (type=8, retval=0x0, file_count=3)
    at /cygdrive/c/Temp/php5-STABLE-200505260839/Zend/zend.c:1084
#15 0x004d6fcd in php_execute_script (primary_file=0x22eed0) at /cygdrive/c/Temp/php5-STABLE-200505260839/main/main.c:1646
#16 0x00536a11 in main (argc=12, argv=0x10012ab0) at /cygdrive/c/Temp/php5-STABLE-200505260839/sapi/cli/php_cli.c:946

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

Just confirming that this happens for php5-STABLE-200506082034 on cygwin / xp-sp2.

still exists in php5-STABLE-200510121840
cygwin DLL version: 1.5.18
libxml2             2.6.20-3