Filtering BBCodes for certain elements

It's easy to explain what I want to achieve, but for me (a novice at PHP), hard to actually achieve it. Basically what I want is to make BBCodes as easily and short as possible. Instead of an array like

$filter=array(
    '[b]'=>'<b>',
    '[/b]'=>'</b>',
    '[i]'=>'<i>',
    '[/i]'=>'</i>');

I'd like to have this array:

$filter=array('b','i');

Then, the part I can't get to, would be where it checks for the strings in that array to have brackets around them (and, another thing I can't figure out, to be able to check also for / in the bracket) and then replace those brackets with <>. So, [b] would become 开发者_JS百科<b> and [/b] would become </b>.

Edit: Solution

function bbcode($string) {
    $filter=array('b','i','u');
    foreach ($filter as $filter) {
        $string=str_replace('['.$filter.']','<'.$filter.'>',$string);
        $string=str_replace('[/'.$filter.']','</'.$filter.'>',$string);
        }
    return $string;
    }

$filter = array('b','i');

$newfilter = array();
foreach ($filter as $tag) {
  $newfilter["[$tag]"] = "<$tag>";
  $newfilter["[/$tag]"] = "</$tag>";
}

Now you can use $newfilter.