Begining with the introduction of PHP 5.3, we have provided you with the ability to upload your own .user.ini files, which are used to modify PHP configuration settings on your account.
If you've used a php.ini file on a linux server, this is the same concept. You simply create a text file named .user.ini in the root of your account. The file can include PHP settings such as:
upload_max_filesize = 200M
date.timezone = "Europe/London"
While most of these settings are safe to change, the .user.ini file does give you a lot of control over how PHP works for your site, so use your best judgement as to what you wish to change.
If you find that you've made a change that causes problems with your site, simply remove the line(s) causing trouble and re-upload the file. Note that there is a five minute server cache for .user.ini changes, so you may not see your changes until a few minutes after uploading.
You can change settings that are labeled PHP_INI_PERDIR, PHP_INI_ALL, and PHP_INI_USER.
You cannot change settings labeled PHP.INI ONLY or PHP_INI_SYSTEM as they are server level settings.