To compile and install you will need libxml. Only versions 2.4.12 and 2.4.14 have been tested. php-soap "should" work with other versions of libxml2, they just haven't been tested
You currently need to check the soap exetension out from Anonymous CVS, using cvs co pear/PECL/soap.
You should copy the soap directory into the PHP /ext folder (unless you only want to build it as a self-contained php-module)
Run phpize in the soap folder (for the module) or ./buildconf in the php4 directory.
Run configure with --with-soap (and your other options)
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