I’ve had some issues myself and got some reports of people who are trying to compile PHP 5.1.2. There error we get is about mulitple definitions of several PCRE lib related symbols and ends with the error that the _xmlTextReaderSchemaValidate is undefined.

/usr/bin/ld: warning multiple definitions of symbol _pcre_callout
ext/pcre/pcrelib/pcre_globals.o definition of _pcre_callout in section (__DATA,__data)
/usr/local/apache2/bin/httpd definition of _pcre_callout
/usr/bin/ld: warning multiple definitions of symbol _pcre_stack_free
ext/pcre/pcrelib/pcre_globals.o definition of _pcre_stack_free in section (__DATA,__data)
/usr/local/apache2/bin/httpd definition of _pcre_stack_free
/usr/bin/ld: warning multiple definitions of symbol _pcre_stack_malloc
ext/pcre/pcrelib/pcre_globals.o definition of _pcre_stack_malloc in section (__DATA,__data)
/usr/local/apache2/bin/httpd definition of _pcre_stack_malloc
/usr/bin/ld: Undefined symbols:
_xmlTextReaderSchemaValidate
collect2: ld returned 1 exit status
make: *** [libs/libphp5.bundle] Error 1

It all works on an Intel based Mac without any problem. If anyone has any idea what causes this problem please let me know !