Many people report problems compiling PHP on Leopard due to a libiconv issue.
This may be due to the small C test programme in ./configure.
It calls open_conv with two const * char arguments (correct), but these are NOT valid charsets to iconv. This then breaks ./configure.
AFAIK from the libiconv documentation, the PHP configure call is incorrect: this function expects two valid charset arguments. (You can see a list of valid arguments by running iconv -l).
Changing the call to:
cd = open_conv("UTF-8","ISO-8859-1")
returns a valid iconv_t number (non-negative), and configure can then continue setting up for your Makefile compile.