PHP preg_replace oddity with £ pound sign and ã - php

Question

I am applying the following function
<?php
function replaceChar($string){
$new_string = preg_replace("/[^a-zA-Z0-9\sçéèêëñòóôõöàáâäåìíîïùúûüýÿ]/", "", $string);
return $new_string;
}
$string = "This is some text and numbers 12345 and symbols !£%^#&$ and foreign letters éèêëñòóôõöàáâäåìíîïùúûüýÿ";
echo replaceChar($string);
?>
which works fine but if I add ã to the preg_replace like
$new_string = preg_replace("/[^a-zA-Z0-9\sçéèêëñòóôõöàáâãäåìíîïùúûüýÿ]/", "", $string);
$string = "This is some text and numbers 12345 and symbols !£%^#&$ and foreign letters éèêëñòóôõöàáâäåìíîïùúûüýÿã";
It conflicts with the pound sign £ and replaces the pound sign with the unidentified question mark in black square.
This is not critical but does anyone know why this is?
Thank you,
Barry
UPDATE: Thank you all. Changed functions adding the u modifier: pt2.php.net/manual/en/… – as suggested by Artefacto and works a treat
function replaceChar($string){
$new_string = preg_replace("/[^a-zA-Z0-9\sçéèêëñòóôõøöàáâãäåìíîïùúûüýÿ]/u", "", $string);
return $new_string;
}

Answer 1

If your string is in UTF-8, you must add the u modifier to the regex. Like this:
function replaceChar($string){
$new_string = preg_replace("/[^a-zA-Z0-9\sçéèêëñòóôõöàáâäåìíîïùúûüýÿ]/u", "", $string);
return $new_string;
}
$string = "This is some text and numbers 12345 and symbols !£%^#&$ and foreign letters éèêëñòóôõöàáâäåìíîïùúûüýÿ";
echo replaceChar($string);

Answer 2

Chances are that your string is UTF-8, but preg_replace() is working on bytes

Answer 3

that code is valid ...
maybe you should try Central-European character encoding
<?php
header ('Content-type: text/html; charset=ISO-8859-2');
?>

Answer 4

You might want to take a look at mb_ereg_replace(). As Mark mentioned preg_replace only works on byte level and does not work well with multibyte character encodings.
Cheers,
Fabian