Problems with instalation

I´m trying install php 4.3.2 but this erro occur [root@mail php-4.3.2]# ./configure --with-apxs2=/usr/local/apache2/bin/apxs --with-mysql checking for ranlib... ranlib checking whether ln -s works... yes checking for gawk... gawk checking for bison... no checking for byacc... no configure: warning: You will need bison if you want to regenerate the PHP parsers. checking for flex... lex checking for yywrap in -ll... no checking lex output file root... ./configure: line 2356: lex: command not found configure: error: cannot find output from lex; giving up [root@mail php-4.3.2]# Who knows what happen? Thanks Vitor