That depends entirely on your os, version of php, and how you have PHP configured, etc. You really have two options for finding it.
1. You can create a test.php and put the following in it:
<?php phpinfo(); ?>
Then view it on the web server through a browser and it will tell you where your error log and php.ini is located.
2. You can look in your php.ini.
In Linux it's usually some where in /etc. My Debian server has it located in /etc/php5/apache2/php.ini
Under Windows it can vary greatly also. On my Windows box mine is located in C:\Program Files\PHP\php.ini