bbcode_convert.php

$str = <<<EOT
Lorem [a href="#"]ipsum[/a] [hr color="red" /] dolor...

[style].foo{color:#fff}[/style]
EOT;

function bbcode_convert($content) {
    // remove style|script
    $content = preg_replace(
        '~(\[(style|sctript)\s?.*\](.*)\[/(\\2)\]|\[(%s)\s?.*/\])~ims', '', $content);
    // remove attributes
    // src,style, <link href> etc..

    $content = preg_replace_callback(
        '~(\[([\w\d]+)(?:\s+(.+)|)\](.+?)\[/([\w\d]+)\])|(\[(\w+)(?:\s+(.+)|)/\])~ims',
    function($m) {
        $attrs = '';
        if (isset($m[8]) && $m[8] != '') $attrs = ' '. trim($m[8]); else
        if (isset($m[3]) && $m[3] != '') $attrs = ' '. trim($m[3]);

        // Self-closing tags
        if (isset($m[6], $m[7])) {
            return sprintf('<%s%s />', $m[7], $attrs);
        }
        // Regular tags
        if (isset($m[2], $m[4], $m[5]) && $m[2] == $m[5]) {
            return sprintf('<%s%s>%s</%s>', $m[2], $attrs, $m[4], $m[2]);
        }
    }, $content);
    return $content;
}