I verified the permissions of archives and modified for 644, and modified the owner and group for apache.nagios.
I verified the 0 variable register_globals and its value is ON. I am using the FC5, with nagios 2.6, php 5, apache 2.0, all packages rpm. I made a program has tested in php to know if exec() obtains to use the function. I verified that the command was not executed, therefore, the problem is in the user who executes the function.
However, already it modifies shell of the apache user. Somebody has some idea of as to change the user to execute the command.