I am using hosting on VPS and they've not allowed me to use php's ini_set function to set configuration at runtime, on a script by script basis. Instead I must change the settings directly in the php.ini file which is a mission to access via SSH.
Why is this?
This, although fine, is slightly annoying as I have 12 domains on the shared server and I do not neccessarily want to set the settings across the board....
VPS providers have a tendency to force their users to use the most secure environment possible. Disabling ini_set() typically is good security practice, since you could easily have an eval() statement in your PHP code that uses a GET/POST/SESSION variable as input, for example. I believe ini_set() is blocked by default if safe_mode is turned on. I would imagine any shared hosting provider would enforce safe_mode.
If it's a product aimed at resellers, it is probably to prevent your clients / customers / users from changing php.ini settings at will, which is usually a wise thing to do.
But if you have full access to the machine, shouldn't you be able to re-activate ini_set?
Like Pekka said, if you can modify your php.ini via SSH, then try to change it there.
Turn off safe_mode and check for contents of disable_functions
This directive allows you to disable certain functions for security reasons. It takes on a comma-delimited list of function names. disable_functions is not affected by Safe Mode. This directive must be set in php.ini For example, you cannot set this in httpd.conf.
Can you tell more about the server components. PHP as CGI or mod_php. Is PHP enabled with suPHP or other protection systems? Some option can be changed with an .htaccess file. Have you direct access to the httpd.conf file? There you can enable ini_set() -> http://php.net/manual/en/configuration.changes.php