I cannot use a different version of php that the one installed.
Free.fr is an ISP that offers free hosting to more than 100,000 sites, on shared servers, with unlimited traffic. It is no question of touching anything, not even the setting of the installed version of PHP.
It is PHP Version 5.1.3RC4-dev, and that is all.
That is why I gave you the link to a full phpinfo. Does Piwik run under this strict implementation of PHP?
Oh sorry, I thought it would be possible to upgrade the PHP using htaccess (like most other hosting do).
I think Piwik should work in 5.1.3, but only trying with latest version, will let us know if it works for sure. Otherwise, I hope free.fr will soon migrate to 5.3 or 5.4 !