$text = "Текст";
$pattern = "/\[b\](.*?)\[\/b\]/i";
$replacement = "<strong>$1</strong>";
$result = preg_replace($pattern, $replacement, $text);
echo $result;
$text = "";
$pattern = "/\[img\](.*?)\[\/img\]/i";
$replacement = "<img src=\"$1\" alt=\"Изображение\">";
$result = preg_replace($pattern, $replacement, $text);
echo $result;
$text = "Текст ссылки";
$pattern = "/\[url=(.*?)\](.*?)\[\/url\]/i";
$replacement = "<a href=\"$1\">$2</a>";
$result = preg_replace($pattern, $replacement, $text);
echo $result;