php - 无法使用str_replace替换html实体

I have the following html entity codes:

$html = "<div id="sstssfb_aaa-theme_wrapper" class="first_sstform_wrapper"> <div class="bscon sstform_wrapper"> <div class="sst_formheaderbgr">"

I want to replace this string sstssfb_aaa-theme_wrapper with the new one using str_replace like so:

$num = 20;

$old = '#sstssfb_aaa-theme_wrapper';
$old = str_replace("#", "", $old);

$new = 'sstssfb_aaa-theme_wrapper' . $num;

$new_html = str_replace($old, $new, $html);

But it won't work at all and returning the original $html instead of the modified.

What would be the problem here and how to solve it?

EDIT:

It works after I trim the $old variable like this:

$old = trim(str_replace("#", "", $old));

Try this

$old = '#sstssfb_aaa-theme_wrapper';
$old = str_replace("#", "", $old);

$new = 'sstssfb_aaa-theme_wrapper' . $num;

$html = str_replace($old, $new, $html);

then printing the $html you will get the result.