php.ini can be located in quite few different places depending on your Linux distribution and Apache installation.

Some common places are:
/etc/php.ini
/etc/php/php.ini
/etc/php5/php.ini
/etc/php5/apache2/php.ini
/usr/bin/php5/bin/php.ini

If you still can’t find it, run:
find / -name php.ini
This will find all files with name php.ini on your server.