Diese Seite bei php.net
The PHP manual text and comments are covered by the Creative Commons Attribution 3.0 License © the PHP Documentation Group - Impressum - mail("TO:Reinhard Neidl",...)
Um diese Funktionen verfügbar zu machen, müssen Sie PHP mit GMP-Support kompilieren. Verwenden Sie dazu die --with-gmp -Option.
With PHP 5.3 on Windows, you need to use the VC9 build, otherwise you will not have the dll to enable GMP
Under Windows you will need to uncomment the line
;extension=php_gmp.dll
within the php.ini and restart Apache.
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.
Diese Seite bei php.net