use replacement- or regex functions. example:
$source = '(b)Hello World(/b) :-)';
$output = $source;
// replace
$output = str_replace( '(b)', '<b>', $output );
$output = str_replace( '(/b)', '</b>', $output );
$output = str_replace( ':-)', '<img src="smiley.gif" height="16" width="16" alt="">', $output );
echo $output;