How to install PHP with the BCMath arbitrary precision math library?
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Due to license restrictions, the BC math library can no longer be distributed
along with PHP.  The BC math library is distributed under the restrictive
GNU Public License, and PHP is no longer distributed under that license, but
under a less restrictive license, the PHP License.

If you want to enable BC math support (recommended), all you have to do is
simply download http://www.php.net/extra/number4.tar.gz, and untar it on the
top level php4 directory.

A typical way of doing this would be (assuming that you have lynx):
cd ~/php4
lynx -dump -source http://www.php.net/extra/number4.tar.gz | gzip -d | tar xvf -

The PHP Group