Where can I find php.ini?

On the command line execute:

php --ini

You will get something like:

Configuration File (php.ini) Path: /etc/php5/cli
Loaded Configuration File:         /etc/php5/cli/php.ini
Scan for additional .ini files in: /etc/php5/cli/conf.d
Additional .ini files parsed:      /etc/php5/cli/conf.d/curl.ini,
/etc/php5/cli/conf.d/pdo.ini,
/etc/php5/cli/conf.d/pdo_sqlite.ini,
/etc/php5/cli/conf.d/sqlite.ini,
/etc/php5/cli/conf.d/sqlite3.ini,
/etc/php5/cli/conf.d/xdebug.ini,
/etc/php5/cli/conf.d/xsl.ini

That's from my local dev-machine. However, the second line is the interesting one. If there is nothing mentioned, have a look at the first one. That is the path, where PHP looks for the php.ini file.

You can grep the same information using phpinfo() in a script and call it with a browser. It’s mentioned in the first block of the output. php -i does the same for the command line, but it’s quite uncomfortable.

The best way to find this is:

Create a PHP (.php) file and add the following code:

<?php phpinfo(); ?>

and open it in a browser. It will show the file which is actually being read!

Updates by the OP:

1. The previously accepted answer is likely to be faster and more convenient for you, but it is not always correct. See comments on that answer.
2. Please also note the more convenient alternative <?php echo php_ini_loaded_file(); ?> mentioned in this answer.