php 正则替换 ubb,php如何将html转成ubb

我的代码如下，为什么html转换ubb无效

$topic_id = "";

$article_id = "";

$uid = "1";

$title = isset($_POST['title']) ? $_POST['title'] : '';

$content = isset($_POST['content']) ? $_POST['content'] : '';

$content = strip_tags($content,',

,,,');//转换html代码

$content = preg_replace('/style=".*?"/', '', $content);

$content = preg_replace("/\s+/", " ", $content);

$content = preg_replace("/

//$content = preg_replace("'([\r\n])[\s]+'", " ", $content);

$content = preg_replace("//", "", $content);//过滤空白标签

$content = preg_replace("/(]*\/>)/", "
\\1
", $content);//前后图片带br

$content = preg_replace("' '", "", $content);

$content = preg_replace("/class=.+?['|\"]/i",'',$content);

$content = eregi_replace(quotemeta("[b]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[/b]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[u]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[/u]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[br]"),quotemeta("
"),$content);

$content = eregi_replace(quotemeta("[/br]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[strong]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[/strong]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[img]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[/img]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[iframe]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[/iframe]"),quotemeta(""),$content);

$content = eregi_replace(quotemeta("[p]"),quotemeta("

"),$content);

$content = eregi_replace(quotemeta("[/p]"),quotemeta("

"),$content);

求解，需要怎么写才能将html转换成ubb代码