Getting certain keywords from a string ?

[eluser]aquary[/eluser]
I'm implementing my webboard using CI and thinking about using some important words from the thread title as a page keyword. I thought about putting those "important words" I want to look for into an array, then foreach and check if any of them exists in the given string.

Code:

$str="Some thread titile with important words";
function get_key($str){

$key=array("important", "words", "I'm", "looking", "for");
$ret='';
foreach($key as $k){
    if(strpos($str, $k)!==FALSE)
        $ret.=$k.', ';
}
return $ret;
}

The function would return "important, words", and I'll put it in the META tag.

The concept should work, but there are too many words I want to put in the array (more that 50, I guess). The script would take some times to return the keywords.

What do you think about this? Are there any other functions that can do this?

Thanks