How to install/enable GMP (math functions) for PHP on Ubuntu?

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http://serverfault.com – When calling a gmp math function like gmp_mod, I get: Fatal error: Call to undefined function gmp_mod() I have installed the php5-gmp package and restarted the web-server, and the function is still undefined. The server is running Ubuntu 10.04 and PHP is running as an Apache module. All software was installed from official packages. How do I enable the GMP math functions in PHP? I'm looking for a solution that uses official packages, not compiling from source. (HowTos)