Problem in make PHP4

Please help me! I install PHP4 follow in web as shown below: root@apl1:/tmp/php-4.0.6 > ./configure --with-mysql --with-imap --with-apache=../apache_1.3.22 --enable-track-vars loading cache ./config.cache checking for a BSD compatible install... /usr/bin/install -c checking whether build environment is sane... yes checking whether make sets ${MAKE}... yes checking for working aclocal... found checking for working autoconf... found checking for working automake... found checking for working autoheader... found checking for working makeinfo... missing checking whether to enable maintainer-specific portions of Makefiles... no checking host system type... i686-pc-linux-gnu checking for gawk... gawk checking for bison... no configure: warning: You will need bison if you want to regenerate the PHP parsers. checking for byacc... no checking for gcc... gcc checking whether the C compiler (gcc ) works... yes checking whether the C compiler (gcc ) is a cross-compiler... no checking whether we are using GNU C... yes checking whether gcc accepts -g... yes checking how to run the C preprocessor... gcc -E checking for AIX... no checking for gcc option to accept ANSI C... none needed checking for ranlib... ranlib checking whether gcc and cc understand -c and -o together... yes checking whether ln -s works... yes 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 And this my problem. What should I do.? Thank you a lot.......