From ../php-4.0.6/ I run :

./configure --with-apache=/usr/local/apache --with-mysql --with-pgsql > error_report.txt

...which produces:

checking lex output file root... ./configure: lex: command not found
configure: error: cannot find output from lex; giving up

...as well as the below output into error_report.txt:

loading cache ./config.cache
checking for a BSD compatible install... /usr/bin/ginstall -c
checking whether build environment is sane... yes
checking whether make sets ${MAKE}... yes
checking for working aclocal... missing
checking for working autoconf... missing
checking for working automake... missing
checking for working autoheader... missing
checking for working makeinfo... found
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
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...