php encoding issue htmlentities - php

Question

I have user input and use htmlentities() to convert all entities.
However, there seems to be some bug. When I type in
ääää öööö üüüü ääää
I get
ääää öööö üüüü ääää
Which looks like this
ääää öööö üüüü ääää
What am I doing wrong? The code is really only this:
$post=htmlentities($post);
EDIT 1
Here is some more code that I use for formatting purposes (there are some helpful functions it them):
//Secure with htmlentities (mysql_real_escape_string() comes later)
$post=htmlentities($post);
//Strip obsolete white spaces
$post = preg_replace("/ +/", " ", $post);
//Detect links
$pattern_url='~(?>[a-z+]{2,}://|www\.)(?:[a-z0-9]+(?:\.[a-z0-9]+)?#)?(?:(?:[a-z](?:[a-z0-9]|(?<!-)-)*[a-z0-9])(?:\.[a-z](?:[a-z0-9]|(?<!-)-)*[a-z0-9])+|(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))(?:/[^\\/:?*"<>|\n]*[a-z0-9])*/?(?:\?[a-z0-9_.%]+(?:=[a-z0-9_.%:/+-]*)?(?:&[a-z0-9_.%]+(?:=[a-z0-9_.%:/+-]*)?)*)?(?:#[a-z0-9_%.]+)?~i';
preg_match_all($pattern_url, $post, $matches);
for ($i=0; $i < count($matches[0]); $i++)
{
if(substr($matches[0][$i],0,4)=='www.')
$post = str_replace($matches[0][$i],'http://'.$matches[0][$i],$post);
}
$post = preg_replace($pattern_url,'<a target="_blank" href="\\0">\\0</a>',$post);
//Keep line breaks (more than one will be stripped above)
$post=nl2br($post);
//Remove more than one linebreak
$post=preg_replace("/(<br\s*\/?>\s*)+/", "<br/>", $post);
//Secure with mysql_real_escape_string()
$post=mysql_real_escape_string($post);

Answer 1

You must manually specify the encoding (UTF-8) for htmlentities():
echo htmlentities("ääää öööö üüüü ääää", null, "UTF-8");
Output:
ääää öööö üüüü ääää

Answer 2

it is important that 3th parameter of htmlentities matches the character set that uses the post. I supouse, you are NOT submiting utf8, as it is the default in htmlentities
in PHP
$post = htmlentities ( $post, ENT_COMPAT, 'ISO-8859-1') // or whatever
in Form
<form action="your.php" accept-charset="ISO-8859-1">
anyway, actualy I recommend you to use utf8