In order to have these functions available, PHP must be compiled with GMP support by using the --with-gmp option.
Note that this parameter requires a path, as in --with-gmp=DIR, if gmp is installed in a non-standard location, which is almost always the case when you are building your own PHP installation in your home directory.
Obvious, but it's an easy mistake.
on ubuntu you can get it from Synaptic Package Manager
just search words "gmp",you'll find it.
on Ubuntu Server:
sudo apt-get install libgmp-dev
sudo apt-get install php5-gmp
sudo service apache2 reload
Under Windows you will need to uncomment the line
;extension=php_gmp.dll
within the php.ini and restart Apache.
On ArchLinux php 5.4.14-1 I had to uncomment the line
extension=gmp.so
in /etc/php/php.ini file.
Probably, restart of web server (e.g. Apache) is required if working from web.
With PHP 5.3 on Windows, you need to use the VC9 build, otherwise you will not have the dll to enable GMP
On Debian platforms, make sure you install the gmp development library first:
sudo apt-get install libgmp-dev
Just like other extensions, you need the libs installed before php compile will gracefully complete.
