<?php

$str = "[Hello!] How is {everyone}?";

$str = preg_replace('~\[([^\]]*)\]~', '<b>\\1</b>', $str);
$str = preg_replace('~{([^}]*)}~', '<i>\\1</i>', $str);

echo $str;

?>

<b>Hello!</b> How is <i>everyone</i>?