What about an optional argument to str_replace which contains the count of replacements...
For example:
<?
$text = "Foobar [b]abc[/b] def [b]ghi";
$tmp_text = str_replace("[b]", "<b>", $text, &$replace_count_open);
$new_text = str_replace("[/b]", "</b>", $text, &$replace_count_close);

if($replace_count_open > $replace_count_close)
    $new_text .= str_repeat("</b>", $replace_count_open - $replace_count_close);
?>

The original definition:
string str_replace (string needle, string str, string haystack)
The definition with this feature:
string str_replace (string needle, string str, string haystack [, int count])