自动把 替换成<p></p>

function nl2p($string, $line_breaks = true, $xml = true)
{
    // Remove existing HTML formatting to avoid double-wrapping things
    $string = str_replace(array('<p>', '</p>', '<br>', '<br />'), '', $string);
    
    // It is conceivable that people might still want single line-breaks
    // without breaking into a new paragraph.
    if ($line_breaks == true)
        return '<p>'.preg_replace(array("/([
]{2,})/i", "/([^>])
([^<])/i"), array("</p>
<p>", '<br'.($xml == true ? ' /' : '').'>'), trim($string)).'</p>';
    else 
        return '<p>'.preg_replace("/([
]{1,})/i", "</p>
<p>", trim($string)).'</p>';
}