Becuase of the way in which we run PHP, you are able to customise its behaviour with php.ini files. These are plain text files (use Notepad if running Windows, TextEdit in Plain Text mode on Mac), which contain directives such as:- 

memory_limit = 24M 

A complete list of php.ini directives can be found here:- 

http://www.php.net/manual/en/ini.list.php 

-: you should only set these as per the requirements of your application as other settings may cause your scripts to malfunction or be vulnerable to attack. 

You should name this file php.ini, unless otherwise required, it should be located in the root of your hosting package (ie, above public_html). 

php.ini files can be used for very fine-grained control - a PHP script will pick up the settings in the php.ini file closest to it, so you can enable functions for a single folder rather than the whole site - this is useful when running CMSes on subdomains.