The libxml extension is enabled by default, although it may be disabled with --disable-libxml .
The optional --with-libxml-dir directive is used to specify the location of libxml on the system that PHP is being compiled on, otherwise only the default locations are scanned. The configure process checks for libxml (specifically, xml2-config) in the following order:
The location ([DIR]) specified with --with-libxml-dir ([DIR]=/bin/xml2-config)
/usr/local/bin/xml2-config
/usr/bin/xml2-config
If configure cannot find xml2-config in the directory specified by --with-libxml-dir , then it'll continue on and check the default locations.
(I tried to enter a bug but the PHP version selection drop down isn't working so I have entered here)
This documentation is very confusing as it refers to xml2-config as a directory and file interchangeably (it is a file) ...so I'm not clear whether to use the containing directory or the actual file in the switch