During make the following exists.  First, the setup:

autoconf : 2.13
bison : (GNU Bison) 1.75

From the output:

bison -y -p ini_ -v -d /cvs/php4/Zend/zend_ini_parser.y -o Zend/zend_ini_parser.c
/cvs/php4/Zend/zend_ini_parser.y:215.4-6: unrecognized escape: `\\0'

PHP compiles but maybe this is something to consider.  It's the only such warning I get.  One thread did talk a little about this specific error but nothing came of it afaict.  Is this a bison bug?

http://marc.theaimsgroup.com/?l=php-dev&m=103680308111343

Line 215 looks like (aside from a little whitespace change):

|  '\0'  { $$.value.str.val = strdup(""); $$.value.str.len=0; $$.type = IS_STRING; }