The libxml extension is enabled by default, although it may be disabled with --without-libxml.
PHP uses pkg-config to select the right library file, header files,
and compile flags to use for libxml2.
To ensure that the desired version of libxml2 is selected,
the PKG_CONFIG_PATH environment variable may
be used to control the search path for pkg-config prior to running the configure script:
PKG_CONFIG_PATH="/path/to/libxml2/prefix/lib/pkgconfig:/lib/pkgconfig"