As mentioned in this new post from the Zend Developer Zone, Nexen.net (creator of the monthly PHP statistics) has kicked off something they call the Month of PHP Functions.
The month of PHP function aims at introducing PHP developers to a new PHP function every day : one that gets too little usage and spotlight, though is highly useful. [...] During this month, we'll present one or several function every day : they all deserve more attention. You don't have to search for it : most of the time, it will be available in your PHP installation, especially if you're using PHP 5.
So far, six functions have already been posted:
Keep an eye on their official page for the project to keep up with their latest function spotlight.