You have to place your custom php.ini file in your Tiki root folder.
Keep in mind that some servers have two different php.ini files, one for php scripts used through a web server and browser, and another one for php scripts being run in the commadn line (CLI). These 2 are common locations for those 2 php.ini files (in Debian GNU/Linux servers):
; if you want to reduce CPU usage, set it to off ; if you want to limit traffic, set to on zlib.output_compression = Off max_execution_time = 60 ; Maximum execution time of each script, in seconds max_input_time = 60 ; Maximum amount of time each script may spend parsing request data memory_limit = 256M ; 128M is PHP's default value. If you still get blank pages, raise it. ; realpath_cache_size determines the size of the cache used by PHP used each time it references a file ; The size value should be increased for Tiki as the default is only 16K ; see http://www.php.net/manual/en/ini.core.php#ini.realpath-cache-size realpath_cache_size = 64k ; The time to live value defaults to 120 seconds so can be increased on stable sites, e.g. ; realpath_cache_ttl = 360
memory_limit=128M upload_max_filesize=10M post_max_size=11M max_execution_time=90
Add on 2015-11-23:
Since php 5.3 you can use .user.ini in stead of php.ini,
Since php 5.5 it is also necessary!! For example on site5.com as they explain here: http://kb.site5.com/php/how-to-make-custom-php-changes-using-a-user-ini-file/ (see the link for more information)