PHP function that produces acronym

I wrote this function because I couldn't find one that produces an acronym from a string of words.

Here's the script:

function acronym($longname) {

$letters=array();

$words=explode(' ', $longname);

foreach($words as $word) {

$word = (substr($word, 0, 1));

array_push($letters, $word);

}

$shortname = strtolower(implode($letters));

return $shortname;

}

// call the script like this:

$_POST['title'] = acronym($_POST['title']);