It seems that PHP versions 4.3.0 and 4.2.3 (mod_php) doubles output buffer inside cusom error handling function.
Let's take simple error handling script:
<?php
function error_handler($errno, $errstr, $errfile, $errline,$errcontext)
{
  echo "123";
  echo "abc";
}

set_error_handler('error_handler');
pg_query('asdf'); // let's trigger error;
?>

I get output "123abc123abc".
Testet on serveral servers/configurations.

my compile options:
Configure Command  './configure' '--with-apxs=/usr/local/sbin/apxs' '--with-config-file-path=/usr/local/etc' '--enable-versioning' '--with-regex=system' '--without-gd' '--without-mysql' '--with-gd=/usr/local' '--enable-gd-native-ttf' '--with-freetype-dir=/usr/local' '--with-jpeg-dir=/usr/local' '--with-png-dir=/usr/local' '--with-zlib' '--with-bz2=/usr' '--with-mcrypt=/usr/local' '--with-mhash=/usr/local' '--with-pdflib=/usr/local' '--with-zlib-dir=/usr' '--with-jpeg-dir=/usr/local' '--with-png-dir=/usr/local' '--with-tiff-dir=/usr/local' '--with-mysql=/usr/local' '--with-pgsql=/usr/local' '--with-openssl=/usr' '--with-snmp=/usr/local' '--enable-ucd-snmp-hack' '--with-openssl=/usr' '--with-expat-dir=/usr/local' '--with-xmlrpc' '--enable-ftp' '--with-curl=/usr/local' '--with-gettext=/usr/local' '--enable-sockets' '--enable-trans-sid' '--with-iconv=/usr/local' '--prefix=/usr/local' 'i386-portbld-freebsd4.7'

flushing buffer before/after triggering error or before/after the err handling ouput doesn't help.