too long

When I use enter in Visual editor on Wordpress, that add   in Text editor, and when I publish the post, the   gives this html code :

<p>�</p>

I think it's a encoded problem ?

With this character my JSON Data content becomes null so I really need to fix that.

I thought about use utf8_decode() but how can I use it ?

I've tried this code, but that doesn't work :

<?php
/* Plugin Name: Erase "�" */
function replace_content($content)
{
$content = str_replace('�', '&nbsp;',$content);
return $content;
}
add_filter('the_content','replace_content')
?>

Maybe I need to use utf8_decode() in this code ? I don't know how to make this...

The website has this html :

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

Maybe it's the problem ? charset = utf-8 twice...