Flex failing on install

Hi people, I have been trying to get PHP4.0.3 to install on my LInux Mandrake 7.1 system and I keep getting an error about Flex not being found. So I went and found an installation of flex and installed that, however I am still getting the same error and the ./configure script keeps giving me the following output: checking for flex... no checking for lex... no ./configure: flex: command not found checking for flex... lex checking for yywrap in -ll... no checking lex output file root... ./configure: lex: command not found configure: error: cannot find output from lex; giving up Is there something that I can do to fix this problem as I needed PHP up and running like 3 days ago :) Any help on this would be greatly appreciated. Thanks -Shannon