The phpinfo() function serves two very helpful purposes:
It replaces the standard "Hello, world!" scripts that verify a PHP installation is working correctly
It outputs a massive amount of very helpful information about a PHP installation, including what is installed and how it is configured.
Unsurprisingly it is the second instance we're most interested in, because phpinfo() outputs information on all extensions enabled in your PHP installation, as well as how they are configured. As such, if you ever want to know a setting, you can read it from php.ini - all the way down to the current level of error reporting, the time that PHP was compiled, and whether or not you have a PHP accelerator installed.